Method for manufacturing semiconductor device comprising oxide semiconductor layer

ABSTRACT

A transistor including an oxide semiconductor film, which has stable electric characteristics is provided. A transistor including an oxide semiconductor film, which has excellent on-state characteristics is also provided. A semiconductor device in which an oxide semiconductor film having low resistance is formed and the resistance of a channel region of the oxide semiconductor film is increased. Note that an oxide semiconductor film is subjected to a process for reducing the resistance to have low resistance. The process for reducing the resistance of the oxide semiconductor film may be a laser process or heat treatment at a temperature higher than or equal to 450° C. and lower than or equal to 740° C., for example. A process for increasing the resistance of the channel region of the oxide semiconductor film having low resistance may be performed by plasma oxidation or implantation of oxygen ions, for example.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an oxide semiconductor film, a methodfor forming the oxide semiconductor film, a semiconductor device, and amethod for manufacturing the semiconductor device.

Note that in this specification, a semiconductor device refers to anydevice that can function by utilizing semiconductor characteristics, andan electro-optical device, a semiconductor circuit, an electronicdevice, and the like are all semiconductor devices.

2. Description of the Related Art

A technique for forming a transistor by using a semiconductor filmformed over a substrate having an insulating surface has attractedattention. The transistor is applied to a wide range of semiconductordevices such as an integrated circuit and a display device. A siliconfilm is known as a semiconductor film applicable to a transistor.

Whether an amorphous silicon film or a polycrystalline silicon film isused as a silicon film in a transistor depends on the purpose. Forexample, in the case of a transistor included in a large-sized displaydevice, it is preferred to use an amorphous silicon film, which can beformed using the established technique for forming a film on alarge-sized substrate. On the other hand, in the case of a transistorincluded in a high-performance display device where driver circuits areformed over the same substrate, it is preferred to use a polycrystallinesilicon film, which can form a transistor having a high field-effectmobility. As a method for forming a polycrystalline silicon film,high-temperature heat treatment or a laser process which is performed onan amorphous silicon film has been known.

Further, in recent years, an oxide semiconductor film has attractedattention. For example, a transistor which includes an amorphous oxidefilm containing indium, gallium, and zinc and having a carrier densityless than 10¹⁸/cm³ is disclosed (see Patent Document 1).

An oxide semiconductor film can be formed by a sputtering method, andthus is suitable for a transistor in a large-sized display device.Moreover, a transistor including an oxide semiconductor film has a highfield-effect mobility; therefore, a high-performance display devicewhere driver circuits are formed over the same substrate can beobtained. In addition, there is an advantage that capital investment canbe reduced because part of production equipment for a transistorincluding an amorphous silicon film can be retrofitted and utilized.

In addition, according to Patent Document 2, a transistor including anoxide semiconductor film can have an extremely low off-state current andcan be favorably used for a semiconductor integrated circuit owing tosuch a low off-state current.

As a method for providing a transistor including an oxide semiconductorfilm with stable electric characteristics, a technique for doping anoxide semiconductor film with oxygen is disclosed (see Patent Document3). With the technique disclosed in Patent Document 3, the impurityconcentration and oxygen vacancies in the oxide semiconductor film canbe reduced. As a result, variation in electric characteristics of thetransistor including the oxide semiconductor film can be reduced andreliability can be improved.

A technique for reducing contact resistance by providing a buffer layerhaving low resistance between a source electrode (or a drain electrode)and the oxide semiconductor film to improve the on-state characteristicsof a transistor including an oxide semiconductor film is also disclosed(see Patent Document 4).

REFERENCE Patent Documents

-   [Patent Document 1] Japanese Published Patent Application No.    2006-165528-   [Patent Document 2] Japanese Published Patent Application No.    2011-151377-   [Patent Document 3] Japanese Published Patent Application No.    2011-243976-   [Patent Document 4] Japanese Published Patent Application No.    2011-9724

SUMMARY OF THE INVENTION

An object is to provide a transistor including an oxide semiconductorfilm, which has stable electric characteristics.

Another object is to provide a transistor including an oxidesemiconductor film, which has excellent on-state characteristics.

One embodiment of the present invention is a method for manufacturing asemiconductor device in which an oxide semiconductor film having lowresistance is formed and the resistance of a channel region of the oxidesemiconductor film is increased.

Note that an oxide semiconductor film is subjected to a process forreducing the resistance to have low resistance. The process for reducingthe resistance of the oxide semiconductor film may be a laser process orheat treatment at a temperature higher than or equal to 450° C. andlower than or equal to 740° C., for example.

Specifically, the heat treatment at a temperature higher than or equalto 450° C. and lower than or equal to 740° C. may be performed in aninert gas atmosphere or under reduced pressure.

The laser process may be performed specifically with an excimer laserwhich emits light having an emission wavelength of 308 nm and an energydensity greater than or equal to 100 mJ/cm² and less than or equal to600 mJ/cm², preferably greater than or equal to 200 mJ/cm² and less thanor equal to 500 mJ/cm², further preferably greater than or equal to 250mJ/cm² and less than or equal to 400 mJ/cm². The conditions of the laserprocess are not limited to the above conditions.

With the laser process or the heat treatment at a temperature higherthan or equal to 450° C. and lower than or equal to 740° C., theimpurity concentration of the oxide semiconductor film can be reducedand an oxygen vacancy can be formed. In addition, the crystallinity ofthe oxide semiconductor film is improved. As a result, an oxidesemiconductor film having low resistance can be obtained.

A process for increasing the resistance of the channel region of theoxide semiconductor film having low resistance may be performed byplasma oxidation or implantation of oxygen ions, for example.

The oxide semiconductor film having low resistance has low impurityconcentration and has many oxygen vacancies. Accordingly, the resistanceof the oxide semiconductor film having low resistance can be increasedby plasma oxidation or implantation of oxygen ions for reducing oxygenvacancies. An oxide semiconductor film having high resistance which isobtained in such a manner is an oxide semiconductor film having lowimpurity concentration and few oxygen vacancies. In other words, sourcesof generating carriers and the like in such an oxide semiconductor filmare extremely reduced. Thus, a transistor including such an oxidesemiconductor film has extremely low off-state current and stableelectric characteristics.

The resistance of a region which is not subjected to the process forincreasing the resistance remains low. Thus, a transistor including theoxide semiconductor film has low parasitic resistance and has excellenton-state characteristics.

One embodiment of the present invention is a method for manufacturing asemiconductor device, including the following steps: forming an oxidesemiconductor film over a substrate having an insulating surface;forming a pair of electrodes over the oxide semiconductor film afterperforming a process for reducing a resistance of the oxidesemiconductor film; forming a gate insulating film over the oxidesemiconductor film and the pair of electrodes after performing a processfor increasing a resistance of a region of the oxide semiconductor film,which does not overlap with the pair of electrodes; and forming a gateelectrode over the oxide semiconductor film with the gate insulatingfilm therebetween.

Another embodiment of the present invention is a method formanufacturing a semiconductor device, including the following steps:forming an oxide semiconductor film over a substrate having aninsulating surface; forming a gate insulating film over the oxidesemiconductor film after performing a process for reducing a resistanceof the oxide semiconductor film; forming a sacrificial layer overlappingwith a part of the oxide semiconductor film with the gate insulatingfilm therebetween; forming a first insulating film over the sacrificiallayer; forming a second insulating film which does not cover a part ofthe gate insulating film; forming a conductive film over the gateinsulating film and the second insulating film after performing aprocess for increasing a resistance of a region of the oxidesemiconductor film, which does not overlap with the second insulatingfilm; and forming a gate electrode by processing the conductive film sothat a top surface of the second insulating film is exposed and a topsurface of the processed conductive film is substantially flush with thetop surface of the second insulating film. The second insulating film isformed in the following manner: a part of the first insulating film isremoved so that a part of a top surface of the sacrificial layer isexposed and a top surface of the first insulating film is substantiallyflush with the part of the top surface of the sacrificial layer; a partof the sacrificial layer and a part of the rest of the first insulatingfilm are removed so that a whole top surface of the sacrificial layer isexposed and a top surface of the sacrificial layer is substantiallyflush with a top surface of the first insulating film; and thesacrificial layer whose whole top surface is exposed is removed.

Another embodiment of the present invention is a method formanufacturing a semiconductor device, including the following steps:forming a gate electrode over a substrate having an insulating surface;forming a gate insulating film over the gate electrode; forming an oxidesemiconductor film over the gate electrode with the gate insulating filmtherebetween; forming a pair of electrodes over the oxide semiconductorfilm after performing a process for reducing a resistance of the oxidesemiconductor film; and performing a process for increasing a resistanceof a region of the oxide semiconductor film, which does not overlap withthe pair of electrodes.

Another embodiment of the present invention is a semiconductor deviceincluding a gate electrode, a gate insulating film in contact with thegate electrode, an oxide semiconductor film overlapping with the gateelectrode with the gate insulating film therebetween, and a pair ofelectrodes over and in contact with the oxide semiconductor film. Acrystallinity of a region of the oxide semiconductor film, whichoverlaps with the pair of electrodes, is higher than a crystallinity ofa region of the oxide semiconductor film, which does not overlap withthe pair of electrodes.

Another embodiment of the present invention is a semiconductor deviceincluding an oxide semiconductor film, a gate insulating film over theoxide semiconductor film, and a gate electrode over the oxidesemiconductor film with the gate insulating film therebetween. Acrystallinity of a region of the oxide semiconductor film, which doesnot overlap with the gate electrode, is higher than a crystallinity of aregion of the oxide semiconductor film, which overlaps with the gateelectrode.

A transistor including an oxide semiconductor film having reducedimpurities and few oxygen vacancies can have stable electriccharacteristics.

A transistor including an oxide semiconductor film having low parasiticresistance can have excellent on-state characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are a top view and cross-sectional views which illustratean example of a transistor of one embodiment of the present invention.

FIGS. 2A to 2C are cross-sectional views illustrating an example of amethod for manufacturing a transistor of one embodiment of the presentinvention.

FIGS. 3A to 3D are cross-sectional views illustrating an example of amethod for manufacturing a transistor of one embodiment of the presentinvention.

FIGS. 4A to 4C are a top view and cross-sectional views which illustratean example of a transistor of one embodiment of the present invention.

FIGS. 5A to 5D are cross-sectional views illustrating an example of amethod for manufacturing a transistor of one embodiment of the presentinvention.

FIGS. 6A to 6D are cross-sectional views illustrating an example of amethod for manufacturing a transistor of one embodiment of the presentinvention.

FIGS. 7A and 7B are cross-sectional views illustrating an example of amethod for manufacturing a transistor of one embodiment of the presentinvention.

FIGS. 8A to 8C are a top view and cross-sectional views which illustratean example of a transistor of one embodiment of the present invention.

FIGS. 9A to 9D are cross-sectional views illustrating an example of amethod for manufacturing a transistor of one embodiment of the presentinvention.

FIGS. 10A to 10D are cross-sectional views illustrating an example of amethod for manufacturing a transistor of one embodiment of the presentinvention.

FIGS. 11A and 11B are a circuit diagram and a cross-sectional view of asemiconductor device of one embodiment of the present invention.

FIGS. 12A and 12B are circuit diagrams of semiconductor devices of oneembodiment of the present invention.

FIGS. 13A and 13B are a circuit diagram and a cross-sectional view of asemiconductor device of one embodiment of the present invention.

FIG. 14 is a circuit diagram of a semiconductor device of one embodimentof the present invention.

FIGS. 15A and 15B are circuit diagrams of a semiconductor device of oneembodiment of the present invention, FIG. 15C is a cross-sectional viewof the semiconductor device, and FIG. 15D is a graph showing electriccharacteristics of the semiconductor device.

FIG. 16A is a circuit diagram of a semiconductor device of oneembodiment of the present invention, FIG. 16B is a graph showingelectric characteristics of the semiconductor device, and FIG. 16C is across-sectional view of the semiconductor device.

FIGS. 17A to 17C are block diagrams illustrating structures of a CPU ofone embodiment of the present invention.

FIG. 18A is a circuit diagram of a display device including an ELelement of one embodiment of the present invention, FIG. 18B is across-sectional view of part of a pixel of the display device, and FIG.18C is a cross-sectional view of a light-emitting layer in the displaydevice.

FIGS. 19A and 19B are a circuit diagram and a cross-sectional view of apixel of a display device including a liquid crystal element of oneembodiment of the present invention.

FIGS. 20A to 20D illustrate electronic devices of one embodiment of thepresent invention.

FIGS. 21A and 21B are graphs each showing the sheet resistance of oxidesemiconductor films after a laser process.

FIG. 22 is a graph showing XRD results of oxide semiconductor filmsafter a laser process.

FIGS. 23A and 23B are cross-sectional TE images of oxide semiconductorfilms after a laser process.

FIGS. 24A and 24B are cross-sectional TE images of oxide semiconductorfilms after a laser process.

FIGS. 25A and 25B are cross-sectional TE images of oxide semiconductorfilms after a laser process.

FIG. 26 is a graph showing the resistance of oxide semiconductor filmsafter a laser process and the resistance of the oxide semiconductorfilms subjected to implantation of oxygen ions.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments and examples of the present invention will be described indetail with reference to the accompanying drawings. However, the presentinvention is not limited to the description below, and it is easilyunderstood by those skilled in the art that modes and details disclosedherein can be modified in a variety of ways. Therefore, the presentinvention is not construed as being limited to description of theembodiments and the examples. In describing structures of the presentinvention with reference to the drawings, the same reference numeralsare used in common for the same portions in different drawings. Notethat the same hatch pattern is applied to similar parts, and the similarparts are not especially denoted by reference numerals in some cases.

Note that a voltage refers to a potential difference between a certainpotential and a reference potential (e.g., a ground potential (GND) or asource potential) in many cases. Accordingly, a voltage can also becalled a potential.

Even when the expression “to be electrically connected” is used in thisspecification, there is a case in which no physical connection is madeand a wiring is just extended in an actual circuit.

Note that the ordinal numbers such as “first” and “second” in thisspecification are used for convenience and do not denote the order ofsteps or the stacking order of layers. In addition, the ordinal numbersin this specification do not denote particular names which specify thepresent invention.

(Embodiment 1)

In this embodiment, a transistor of one embodiment of the presentinvention and a method for manufacturing the transistor will bedescribed.

FIG. 1A is a top view of a transistor of one embodiment of the presentinvention. FIG. 1B is a cross-sectional view along dashed-dotted lineA1-A2 of FIG. 1A. FIG. 1C is a cross-sectional view along dashed-dottedline A3-A4 of FIG. 1A. Note that a gate insulating film 112 and the likeare not illustrated in FIG. 1A for simplicity.

The transistor illustrated in FIG. 1B includes a base insulating film102 over a substrate 100; an oxide semiconductor film 106 which includesa low-resistance region 106 a, a low-resistance region 106 b, and ahigh-resistance region 106 c and is over the base insulating film 102; asource electrode 116 a and a drain electrode 116 b in contact with thelow-resistance region 106 a and the low-resistance region 106 b,respectively; the gate insulating film 112 over the oxide semiconductorfilm 106, the source electrode 116 a, and the drain electrode 116 b; anda gate electrode 104 over the oxide semiconductor film 106 with the gateinsulating film 112 provided therebetween. Although the transistorillustrated in FIG. 1B includes the base insulating film 102, thepresent invention is not limited thereto. For example, the baseinsulating film 102 is not necessarily provided. Note that the sourceelectrode 116 a and the drain electrode 116 b may be collectivelyreferred to as a pair of electrodes.

Specifically, the low-resistance region 106 a and the low-resistanceregion 106 b included in the oxide semiconductor film 106 overlap withthe source electrode 116 a and the drain electrode 116 b, respectively,in the top view of FIG. 1A. The high-resistance region 106 c included inthe oxide semiconductor film 106 overlaps with neither the sourceelectrode 116 a nor the drain electrode 116 b in the top view of FIG.1A. Thus, the low-resistance region 106 a and the low-resistance region106 b of the oxide semiconductor film 106 function as a source regionand a drain region of the transistor, respectively. Further, part of thehigh-resistance region 106 c of the oxide semiconductor film 106 (aregion between the source electrode 116 a and the drain electrode 116 b)functions as a channel region of the transistor.

The low-resistance region 106 a and the low-resistance region 106 bincluded in the oxide semiconductor film 106 have higher crystallinitythan the high-resistance region 106 c included in the oxidesemiconductor film 106. It is particularly preferable that thelow-resistance region 106 a and the low-resistance region 106 b includedin the oxide semiconductor film 106 include polycrystalline regions. Thehigh-resistance region 106 c included in the oxide semiconductor film106 may include an amorphous region.

The low-resistance region 106 a and the low-resistance region 106 bincluded in the oxide semiconductor film 106 may be formed using ac-axis aligned crystalline oxide semiconductor (CAAC-OS).

In most cases, a crystal part in the CAAC-OS fits inside a cube whoseone side is less than 100 nm In an image obtained with a transmissionelectron microscope (TEM), a boundary between crystal parts in theCAAC-OS is not clearly detected. Further, with the TEM, a grain boundaryin the CAAC-OS is not clearly found. Thus, in the CAAC-OS, a reductionin electron mobility due to the grain boundary is suppressed.

In each of the crystal parts included in the CAAC-OS, for example, ac-axis is aligned in a direction parallel to a normal vector of asurface where the oxide semiconductor film is formed or a normal vectorof a surface of the CAAC-OS film. Further, in each of the crystal parts,metal atoms are arranged in a triangular or hexagonal configuration whenseen from the direction perpendicular to the a-b plane, and metal atomsare arranged in a layered manner or metal atoms and oxygen atoms arearranged in a layered manner when seen from the direction perpendicularto the c-axis. Note that, among crystal parts, the directions of thea-axis and the b-axis of one crystal part may be different from those ofanother crystal part. In this specification, a term “perpendicular”includes a range from 80° to 100°, preferably from 85° to 95°. Inaddition, a term “parallel” includes a range from −10° to 10°,preferably from −5° to 5°.

For the oxide semiconductor film 106, an In-M-Zn oxide may be used, forexample. Here, a metal element M is an element whose bond energy withoxygen is higher than that of In and that of Zn. Alternatively, themetal element M is an element which has a function of suppressingelimination of oxygen from the In-M-Zn oxide. Owing to the effect of themetal element M, generation of oxygen vacancies in the oxidesemiconductor film 106 is suppressed. Note that oxygen vacancies in theoxide semiconductor film 106 generate carriers in some cases. Thus, theeffect of the metal element M suppresses an increase in the carrierdensity of the oxide semiconductor film 106 and an increase in off-statecurrent. Furthermore, a change in the electrical characteristics of thetransistor, which is caused by oxygen vacancies, can be reduced, wherebya highly reliable transistor can be obtained.

The metal element M can be, specifically, Al, Sc, Ti, V, Cr, Mn, Fe, Co,Ni, Ga, Y, Zr, Nb, Mo, Sn, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er,Tm, Yb, Lu, Hf, Ta, or W, and is preferably Al, Ti, Ga, Y, Zr, Ce, orHf. As the metal element M, one or more elements of the above elementsmay be selected. Further, Si or Ge may be used instead of the metalelement M.

The hydrogen concentration in the high-resistance region 106 c includedin the oxide semiconductor film 106 is lower than or equal to 2×10²⁰atoms/cm³, preferably lower than or equal to 5×10¹⁹ atoms/cm³, and morepreferably lower than or equal to 1×10¹⁹ atoms/cm³. This is becausehydrogen included in the oxide semiconductor film generatesunintentional carriers in some cases. The generated carriers mightincrease the off-state current of the transistor and vary the electricalcharacteristics of the transistor. Thus, when the hydrogen concentrationin the high-resistance region 106 c, which is the channel region of thetransistor is in the above range, an increase in the off-state currentof the transistor and a change in the electrical characteristics of thetransistor can be suppressed.

The oxide semiconductor film 106 has a wider band gap than a siliconfilm by approximately 1 eV to 2 eV. For that reason, in the transistorincluding the oxide semiconductor film 106, impact ionization isunlikely to occur and avalanche breakdown is unlikely to occur. That is,it can be said that, in the transistor including the oxide semiconductorfilm 106, hot-carrier degradation is unlikely to occur.

Furthermore, there are a small number of sources of generating carriersin the high-resistance region 106 c included in the oxide semiconductorfilm 106; accordingly, the channel region can be completely depleted byan electric field of the gate electrode 104 even in the case where theoxide semiconductor film 106 has a large thickness (for example, greaterthan or equal to 15 nm and less than 100 nm). For that reason, in thetransistor in which the high-resistance region 106 c included in theoxide semiconductor film 106 is a channel region, an increase inoff-state current and a change in threshold voltage due to apunch-through phenomenon are not caused.

The oxygen vacancies in the oxide semiconductor film can be evaluated byelectron spin resonance (ESR). That is, an oxide semiconductor filmwithout an oxygen vacancy (or with few oxygen vacancies) can be referredto as an oxide semiconductor film which does not have a signal due tooxygen vacancies evaluated by ESR. Specifically, the spin densityattributed to oxygen vacancies in the high-resistance region 106 cincluded in the oxide semiconductor film 106 is lower than 5×10¹⁶spins/cm³. When the oxide semiconductor film has oxygen vacancies, asignal having symmetry is found at a g value of around 1.93 in ESR.

A transistor including the oxide semiconductor film can have anextremely low off-state current by a significant reduction inconcentration of a donor (e.g., hydrogen or an oxygen vacancy) in theoxide semiconductor film. Specifically, the off-state current of atransistor with a channel length of 3 μm and a channel width of 1 μm canbe lower than or equal to 1×10⁻²¹ A or lower than or equal to 1×10⁻²⁵ A.

There is no particular limitation on the substrate 100. For example, aglass substrate, a ceramic substrate, a quartz substrate, or a sapphiresubstrate may be used as the substrate 100. Alternatively, a singlecrystal semiconductor substrate or a polycrystalline semiconductorsubstrate of silicon, silicon carbide, or the like, a compoundsemiconductor substrate silicon germanium or the like, asilicon-on-insulator (SOI) substrate, or the like may be used. Stillalternatively, any of these substrates provided with a semiconductorelement may be used as the substrate 100.

In the case of using a large glass substrate such as the fifthgeneration (1000 mm×1200 mm or 1300 mm×1500 mm); the sixth generation(1500 mm×1800 mm); the seventh generation (1870 mm×2200 mm); the eighthgeneration (2200 mm×2500 mm); the ninth generation (2400 mm×2800 mm); orthe tenth generation (2880 mm×3130 mm) as the substrate 100,microfabrication is difficult in some cases due to the shrinkage of thesubstrate 100, which is caused by heat treatment or the like in amanufacturing process of the semiconductor device. Therefore, in thecase where the above-described large glass substrate is used as thesubstrate 100, a substrate which is unlikely to shrink through the heattreatment is preferably used. For example, a large-sized glass substratewhich has a shrinkage of 10 ppm or less, preferably 5 ppm or less,further preferably 3 ppm or less after heat treatment at 400° C.,preferably at 450° C., and more preferably 500° C. for one hour may beused as the substrate 100.

Further alternatively, a flexible substrate may be used as the substrate100. Note that as a method for providing a transistor over a flexiblesubstrate, there is a method in which, after a transistor is formed overa non-flexible substrate, the transistor is separated from thenon-flexible substrate and transferred to the substrate 100 which is aflexible substrate. In that case, a separation layer is preferablyprovided between the non-flexible substrate and the transistor.

As the base insulating film 102, a single layer of an insulating filmsuch as a silicon oxide film, a silicon oxynitride film, a siliconnitride oxide film, a silicon nitride film, an aluminum oxide film, ahafnium oxide film, an yttrium oxide film, a zirconium oxide film, agallium oxide film, a tantalum oxide film, a magnesium oxide film, alanthanum oxide film, a cerium oxide film, or a neodymium oxide film ora stack of any of these insulating films is used. It is preferable touse a single layer of an insulating film such as a silicon oxide film, asilicon oxynitride film, a silicon nitride oxide film, or a siliconnitride film or a stack of any of these insulating films.

The base insulating film 102 preferably contains excess oxygen in aregion which overlaps with the high-resistance region 106 c included inthe oxide semiconductor film 106.

An insulating film containing excess oxygen can release oxygen by beingsubjected to heat treatment or the like. That is, an insulating filmcontaining excess oxygen has a function of releasing oxygen by beingsubjected to heat treatment.

In the case where the base insulating film 102 contains excess oxygen inthe region which overlaps with the high-resistance region 106 c includedin the oxide semiconductor film 106, oxygen vacancies in thehigh-resistance region 106 c included in the oxide semiconductor film106 can be reduced.

Here, to release oxygen by heat treatment means that the amount ofreleased oxygen which is converted into oxygen atoms is greater than orequal to 1×10¹⁸ atoms/cm³, greater than or equal to 1×10¹⁹ atoms/cm³, orgreater than or equal to 1×10²⁰ atoms/cm³ in thermal desorptionspectroscopy (TDS) analysis.

A method for measuring the amount of released oxygen using TDS isdescribed below.

The total amount of released gas from a measurement sample in TDS isproportional to the integral value of the ion intensity of the releasedgas. Then, a comparison with a standard sample is made, whereby thetotal amount of the released gas can be calculated.

For example, the number of released oxygen molecules (N_(O2)) from ameasurement sample can be calculated according to Formula (1) using theTDS results of a silicon wafer containing hydrogen at a predetermineddensity, which is the standard sample, and the TDS results of themeasurement sample. Here, all gasses having a mass number of 32 whichare obtained by the TDS are assumed to originate from an oxygenmolecule. CH₃OH can also be given as a gas having a mass number of 32,but is not taken into consideration on the assumption that CH₃OH isunlikely to be present. Further, an oxygen molecule including an oxygenatom having a mass number of 17 or 18, which is an isotope of an oxygenatom, is also not taken into consideration because the proportion ofsuch a molecule in the natural world is minimal.

$\begin{matrix}{\left\lbrack {{FORMULA}\mspace{14mu} 1} \right\rbrack\mspace{585mu}} & \; \\{N_{O\; 2} = {\frac{N_{H\; 2}}{S_{H\; 2}} \times S_{O\; 2} \times \alpha}} & (1)\end{matrix}$

The value N_(H2) is obtained by conversion of the number of hydrogenmolecules desorbed from the standard sample into density. The integralvalue of ion intensity when the standard sample is analyzed by TDS isdenoted by S_(H2). Here, the reference value of the standard sample isexpressed by N_(H2)/S_(H2). The integral value of ion intensity when themeasurement sample is analyzed by TDS is denoted by S_(O2), and thecoefficient affecting the ion intensity in the TDS is denoted by α. Forthe details of Formula (1), refer to Japanese Published PatentApplication No. H6-275697. Note that the amount of released oxygen wasmeasured with a thermal desorption spectroscopy apparatus produced byESCO Ltd., EMD-WA1000S/W using a silicon wafer containing hydrogen atomsat 1×10¹⁶ atoms/cm² as the standard sample.

Further, in the TDS, oxygen is partly detected as an oxygen atom. Theratio between oxygen molecules and oxygen atoms can be calculated fromthe ionization rate of oxygen molecules. Note that, since the above α isdetermined considering the ionization rate of oxygen molecules, thenumber of released oxygen atoms can be estimated through the evaluationof the number of the released oxygen molecules.

Note that N_(O2) is the number of released oxygen molecules. When thenumber of released oxygen molecules is converted into the number ofreleased oxygen atoms, the number of released oxygen atoms is twice thenumber of released oxygen molecules.

The insulating film from which oxygen is released by heat treatment isan insulating film containing a peroxide radical. Specifically, the spindensity attributed to a peroxide radical of the insulating film ishigher than or equal to 5×10¹⁷ spins/cm³. Note that the insulating filmcontaining a peroxide radical has a signal having asymmetry at a g valueof around 2.01 in ESR.

The insulating film containing excess oxygen may be formed usingoxygen-excess silicon oxide (SiO_(x) (X>2)). In the oxygen-excesssilicon oxide (SiO_(x) (X>2)), the number of oxygen atoms per unitvolume is more than twice the number of silicon atoms per unit volume.The number of silicon atoms and the number of oxygen atoms per unitvolume are measured by RBS.

The source electrode 116 a and the drain electrode 116 b may be formedof a single layer or a stacked layer of a conductive film of a simplesubstance selected from aluminum, titanium, chromium, cobalt, nickel,copper, yttrium, zirconium, molybdenum, silver, tantalum, and tungsten;a nitride containing one or more kinds of the above substances; an oxidecontaining one or more kinds of the above substances; or an alloycontaining one or more kinds of the above substances. Note that thesource electrode 116 a and the drain electrode 116 b may have the samecomposition or different compositions.

As the gate insulating film 112, a single layer of an insulating filmsuch as a silicon oxide film, a silicon oxynitride film, a siliconnitride oxide film, a silicon nitride film, an aluminum oxide film, ahafnium oxide film, an yttrium oxide film, a zirconium oxide film, agallium oxide film, a tantalum oxide film, a magnesium oxide film, alanthanum oxide film, a cerium oxide film, or a neodymium oxide film ora stack of any of these insulating films is used. It is preferable touse a single layer of an insulating film such as a silicon oxide film, asilicon oxynitride film, a silicon nitride oxide film, or a siliconnitride film or a stack of any of these insulating films.

Note that the gate insulating film 112 preferably contains excessoxygen.

In the case where the gate insulating film 112 contains excess oxygen,oxygen vacancies in the high-resistance region 106 c included in theoxide semiconductor film 106 can be reduced.

The gate electrode 104 may be formed of a single layer or a stackedlayer of a conductive film of a simple substance selected from aluminum,titanium, chromium, cobalt, nickel, copper, yttrium, zirconium,molybdenum, silver, tantalum, and tungsten; a nitride containing one ormore kinds of the above substances; an oxide containing one or morekinds of the above substances; or an alloy containing one or more kindsof the above substances.

In the aforementioned transistor, the low-resistance region 106 a andthe low-resistance region 106 b which are included in the oxidesemiconductor film 106 have high crystallinity. Thus, the transistor haslow parasitic resistance and excellent on-state characteristics.Further, since the high-resistance region 106 c included in the oxidesemiconductor film 106 has low impurity concentration and few oxygenvacancies, the transistor has stable electric characteristics.

Next, a method for manufacturing the transistor illustrated in FIGS. 1Ato 1C will be described. Note that the method for manufacturing thetransistor will be described with reference to cross-sectional viewseach corresponding to FIG. 1B.

First, the base insulating film 102 is formed over the substrate 100(see FIG. 2A). The base insulating film 102 can be formed using aninsulating film selected from the insulating films given as examples ofthe base insulating film 102 and can be formed by a sputtering method, achemical vapor deposition (CVD) method, a molecular beam epitaxy (MBE)method, an atomic layer deposition (ALD) method, or a pulsed laserdeposition (PLD) method.

Then, an oxide semiconductor film 136 a is formed (see FIG. 2B). Theoxide semiconductor film 136 a may be formed using an oxide filmselected from the oxide films given as examples of the oxidesemiconductor film 106 and may be formed by a sputtering method, a CVDmethod, an MBE method, an ALD method, or a PLD method.

Subsequently, a process for reducing the resistance of the oxidesemiconductor film 136 a is performed to form an oxide semiconductorfilm 136 b having low resistance (see FIG. 2C). The process for reducingthe resistance of the oxide semiconductor film 136 a may be a laserprocess or heat treatment at a temperature higher than or equal to 450°C. and lower than or equal to 740° C., for example.

Specifically, the heat treatment at a temperature higher than or equalto 450° C. and lower than or equal to 740° C. may be performed in aninert gas atmosphere or under reduced pressure. The heating time may belonger than or equal to 1 minute and shorter than or equal to 24 hours,preferably longer than or equal to 6 minutes and shorter than or equalto 5 hours, further preferably longer than or equal to 30 minutes andshorter than or equal to 2 hours.

The laser process may be performed specifically with an excimer laserwhich emits light having an emission wavelength of 308 nm and an energydensity greater than or equal to 100 mJ/cm² and less than or equal to600 mJ/cm², preferably greater than or equal to 200 mJ/cm² and less thanor equal to 500 mJ/cm², further preferably greater than or equal to 250mJ/cm² and less than or equal to 400 mJ/cm². The conditions of the laserprocess are not limited to the above conditions. In principle, a laseremitting light whose energy converted from the emission wavelength ishigher than the band gap (about 2 eV to 4 eV) of the oxide semiconductorfilm to be subjected to the laser process may be used. For example, anexcimer laser which emits light having an emission wavelength of 193 nm,248 nm, or 351 nm may be used. Alternatively, a semiconductor laser, asolid-state laser, or the like may be used. Note that since thefundamental wavelengths of a semiconductor laser and a solid-state laserare large, a harmonic is used.

When the laser process is performed, the shrinkage of the substrate 100can be practically reduced even when heat treatment at extremely hightemperature is performed. With the laser process performed only on anintended region in the substrate 100, the time needed for the laserprocess can be shortened and productivity can be enhanced. Note that thesubstrate 100 may be subjected to the laser process while being heated.

With the laser process or the heat treatment at a temperature higherthan or equal to 450° C. and lower than or equal to 740° C., theimpurity concentration of the oxide semiconductor film 136 a can bereduced and an oxygen vacancy can be formed. In addition, thecrystallinity is improved. As a result, an oxide semiconductor film 136b having low resistance can be obtained.

Next, the oxide semiconductor film 136 b is processed into anisland-shaped oxide semiconductor film 136.

Then, a conductive film to be the source electrode 116 a and the drainelectrode 116 b is formed. The conductive film can be selected from theconductive films given as examples of the source electrode 116 a and thedrain electrode 116 b and can be formed by a sputtering method, a CVDmethod, an MBE method, an ALD method, or a PLD method.

Then, a conductive film to be the source electrode 116 a and the drainelectrode 116 b is processed to form the source electrode 116 a and thedrain electrode 116 b (see FIG. 3A).

After that, a region of the oxide semiconductor film 136, which overlapswith neither the source electrode 116 a nor the drain electrode 116 b,is subjected to a process for increasing the resistance of the region ofthe oxide semiconductor film.

A process for increasing the resistance of the oxide semiconductor film136 may be performed by plasma oxidation or implantation of oxygen ions,for example. In this embodiment, oxygen ions 130 are implanted (see FIG.3B).

The oxide semiconductor film 136 having low resistance has low impurityconcentration and has many oxygen vacancies. Accordingly, the resistanceof the oxide semiconductor film having low resistance can be increasedby plasma oxidation or implantation of oxygen ions for reducing oxygenvacancies. An oxide semiconductor film having high resistance which isobtained in such a manner is an oxide semiconductor film having lowimpurity concentration and few oxygen vacancies.

The process for increasing the resistance of the oxide semiconductorfilm 136 enables the region of the oxide semiconductor film 136, whichoverlaps with neither the source electrode 116 a nor the drain electrode116 b, to be a high-resistance region. Meanwhile, regions of the oxidesemiconductor film 136, which overlap with the source electrode 116 aand the drain electrode 116 b, are low-resistance regions. In thismanner, the oxide semiconductor film 106 including the low-resistanceregion 106 a, the low-resistance region 106 b, and the high-resistanceregion 106 c can be formed (see FIG. 3C).

Note that the process for increasing the resistance of the oxidesemiconductor film 136 might oxidize surfaces of the source electrode116 a and the drain electrode 116 b. An increase in parasitic resistancedue to the oxidation of the surfaces of the source electrode 116 a andthe drain electrode 116 b can be ignored when the source electrode 116 aand the drain electrode 116 b have sufficient thicknesses. Specifically,the thicknesses of the source electrode 116 a and the drain electrode116 b are greater than or equal to 50 nm, preferably greater than orequal to 100 nm.

In some cases, the oxidation of the surfaces of the source electrode 116a and the drain electrode 116 b can prevent leakage current between thegate electrode 104 and the source electrode 116 a and between the gateelectrode 104 and the drain electrode 116 b.

A region of the base insulating film 102, which overlaps with thehigh-resistance region 106 c included in the oxide semiconductor film106, may contain excess oxygen by the process for increasing theresistance of the oxide semiconductor film 136 or by a similar process.

Next, the gate insulating film 112 is formed. The gate insulating film112 can be formed using an insulating film selected from the insulatingfilms given as examples of the gate insulating film 112 and can beformed by a sputtering method, a CVD method, an MBE method, an ALDmethod, or a PLD method.

The process for increasing the resistance of the oxide semiconductorfilm 136 can be performed after the formation of the gate insulatingfilm 112 instead of after the formation of the source electrode 116 aand the drain electrode 116 b. In that case, a region of the gateinsulating film 112 or the region of the base insulating film 102, whichoverlaps with the high-resistance region 106 c included in the oxidesemiconductor film 106, may contain excess oxygen by the process forincreasing the resistance of the oxide semiconductor film 136 or by asimilar process.

Then, a conductive film to be the gate electrode 104 is formed. Theconductive film can be selected from the conductive films given asexamples of the gate electrode 104 and can be formed by a sputteringmethod, a CVD method, an MBE method, an ALD method, or a PLD method.

After that, the conductive film to be the gate electrode 104 isprocessed to form the gate electrode 104 (see FIG. 3D).

Through the above steps, the transistor illustrated in FIGS. 1A to 1Ccan be manufactured.

Regions which function as the source region and the drain region of thetransistor have low resistance. Thus, a transistor including the oxidesemiconductor film has low parasitic resistance and has excellenton-state characteristics. Further, sources of generating carriers andthe like in the channel region of the oxide semiconductor film areextremely reduced. Thus, a transistor including such an oxidesemiconductor film has low off-state current and stable electriccharacteristics.

This embodiment can be implemented in appropriate combination with anyof the other embodiments and examples.

(Embodiment 2)

In this embodiment, a transistor having a structure different from thestructures of the transistors in Embodiment 1 and a method ofmanufacturing the transistor will be described.

FIG. 4A is a top view of a transistor of one embodiment of the presentinvention. FIG. 4B is a cross-sectional view along dashed-dotted lineB1-B2 of FIG. 4A. FIG. 4C is a cross-sectional view along dashed-dottedline B3-B4 of FIG. 4A. Note that a gate insulating film 212 and the likeare not illustrated in FIG. 4A for simplicity.

The transistor illustrated in FIG. 4B includes a base insulating film202 over a substrate 200; an oxide semiconductor film 206 which includesa low-resistance region 206 a, a low-resistance region 206 b, and ahigh-resistance region 206 c and is over the base insulating film 202;the gate insulating film 212 over the oxide semiconductor film 206; agate electrode 204 over the high-resistance region 206 c included in theoxide semiconductor film 206 with the gate insulating film 212 providedtherebetween; and an insulating film 218 which is over the gateinsulating film 212. The top surfaces of the insulating film 218 and thegate electrode 204 are substantially flush with each other. Although thetransistor illustrated in FIG. 4B includes the base insulating film 202,the present invention is not limited thereto. For example, the baseinsulating film 202 is not necessarily provided.

Specifically, the low-resistance region 206 a and the low-resistanceregion 206 b included in the oxide semiconductor film 206 do not overlapwith the gate electrode 204. The high-resistance region 206 c includedin the oxide semiconductor film 206 overlaps with the gate electrode204. Thus, the low-resistance region 206 a and the low-resistance region206 b of the oxide semiconductor film 206 function as a source regionand a drain region of the transistor, respectively. Further, part of thehigh-resistance region 206 c of the oxide semiconductor film 206functions as a channel region of the transistor.

For the oxide semiconductor film 206, refer to the description of theoxide semiconductor film 106. Further, for the low-resistance region 206a, the low-resistance region 206 b, and the high-resistance region 206 cincluded in the oxide semiconductor film 206, refer to the descriptionsof the low-resistance region 106 a, the low-resistance region 106 b, andthe high-resistance region 106 c included in the oxide semiconductorfilm 106.

For the substrate 200, refer to the description of the substrate 100.

For the base insulating film 202, refer to the description of the baseinsulating film 102.

For the gate insulating film 212, refer to the description of the gateinsulating film 112.

For the gate electrode 204, refer to the description of the gateelectrode 104.

As the insulating film 218, a single layer of an insulating film such asa silicon oxide film, a silicon oxynitride film, a silicon nitride oxidefilm, a silicon nitride film, an aluminum oxide film, a hafnium oxidefilm, an yttrium oxide film, a zirconium oxide film, a gallium oxidefilm, a tantalum oxide film, a magnesium oxide film, a lanthanum oxidefilm, a cerium oxide film, or a neodymium oxide film or a stack of anyof these insulating films is used. It is preferable to use a singlelayer of an insulating film such as a silicon oxide film, a siliconoxynitride film, a silicon nitride oxide film, or a silicon nitride filmor a stack of any of these insulating films.

Next, a method for manufacturing the transistor illustrated in FIGS. 4Ato 4C will be described. Note that the method for manufacturing thetransistor will be described with reference to cross-sectional viewseach corresponding to FIG. 4B.

First, the base insulating film 202 is formed over the substrate 200.For the base insulating film 202, refer to the description of the methodfor forming the base insulating film 102.

Next, an oxide semiconductor film 236 is formed. For the oxidesemiconductor film 236, refer to the description of the method forforming the oxide semiconductor film 136.

Then, the gate insulating film 212 is formed (see FIG. 5A). For the gateinsulating film 212, refer to the description of the method for formingthe gate insulating film 112.

Subsequently, an insulating film, a semiconductor film, or a conductivefilm, which is to be a sacrificial layer 234 is formed. The insulatingfilm, the semiconductor film, or the conductive film, which is to be thesacrificial layer 234 may be formed by a sputtering method, a CVDmethod, an MBE method, an ALD method, or a PLD method. Note that thesacrificial layer 234 can be any kind of film, such as an insulatingfilm, a semiconductor film, or a conductive film, as long as the filmcan be selectively etched from an insulating film 248 (which is to bethe insulating film 218) formed later. In this embodiment, an insulatingfilm, a semiconductor film, or a conductive film, which is to be thesacrificial layer 234, selected from the conductive films given asexamples of the gate electrode 104 is used.

Next, the conductive film which is to be the sacrificial layer 234 isprocessed to form the sacrificial layer 234 (see FIG. 5B).

After that, the insulating film 248 is formed (see FIG. 5C). Theinsulating film 248 can be formed using an insulating film selected fromthe insulating films given as examples of the insulating film 218 andcan be formed by a sputtering method, a CVD method, an MBE method, anALD method, or a PLD method. It is preferable that the thickness of theinsulating film 248 be larger than the thickness of the sacrificiallayer 234 because later steps can be easily performed.

Then, part of the insulating film 248 is removed so that the top surfaceof the insulating film 248 is level and part of the top surface of thesacrificial layer 234 is exposed. After that, parts of the insulatingfilm 248 and the sacrificial layer 234 are removed so that the topsurfaces of the insulating film 248 and the sacrificial layer 234 aresubstantially flush with each other; thus, a sacrificial layer 235 whosewhole top surface is exposed and an insulating film 249 whose topsurface is substantially flush with that of the sacrificial layer 235are formed (see FIG. 5D).

As a method for removing parts of the insulating film 248 and thesacrificial layer 234 so that the top surfaces of the insulating film248 and the sacrificial layer 234 are substantially flush with eachother, a chemical mechanical polishing (CMP) process may be used.Alternatively, a planarization film may be formed so that its topsurface is level and then be etched from the top with the etching ratecontrolled.

Next, the sacrificial layer 235 is removed by an etching process underconditions that the sacrificial layer 235 is etched, but the insulatingfilm 249 is not etched (see FIG. 6A). By removal of the sacrificiallayer 235, the insulating film 249 exposes part of the gate insulatingfilm 212.

Note that part of the gate insulating film 212 may be removed togetherwith the sacrificial layer 235.

Subsequently, a region of the oxide semiconductor film 236, which doesnot overlap with the insulating film 249, is subjected to a process forincreasing the resistance of the oxide semiconductor film.

A process for increasing the resistance of the oxide semiconductor film236 may be performed by plasma oxidation or implantation of oxygen ions,for example. In this embodiment, oxygen ions 230 are implanted (see FIG.6B).

The oxide semiconductor film 236 having low resistance has low impurityconcentration and has many oxygen vacancies. Accordingly, the resistanceof the oxide semiconductor film having low resistance can be increasedby plasma oxidation or implantation of oxygen ions for reducing oxygenvacancies. An oxide semiconductor film having high resistance which isobtained in such a manner is an oxide semiconductor film having lowimpurity concentration and few oxygen vacancies.

The process for increasing the resistance of the oxide semiconductorfilm 236 enables the region of the oxide semiconductor film 236, whichdoes not overlap with the insulating film 249, to be a high-resistanceregion. Meanwhile, regions of the oxide semiconductor film 236, whichoverlap with the insulating film 249, are low-resistance regions. Inthis manner, the oxide semiconductor film 206 including thelow-resistance region 206 a, the low-resistance region 206 b, and thehigh-resistance region 206 c can be formed (see FIG. 6C).

Note that a region of the gate insulating film 212, which overlaps withthe high-resistance region 206 c included in the oxide semiconductorfilm 206, or a region of the base insulating film 202, which overlapswith the high-resistance region 206 c included in the oxidesemiconductor film 206, may contain excess oxygen by the process forincreasing the resistance of the oxide semiconductor film 236 or by asimilar process.

After that, part of the gate insulating film 212 may be removed.

In the case where the part of the gate insulating film 212 is removedtogether with the sacrificial layer 235 or in the case where the part ofthe gate insulating film 212 is removed after the process for increasingthe resistance of the oxide semiconductor film 236, a gate insulatingfilm is formed at this time.

Then, a conductive film 205 is formed (see FIG. 6D). For the conductivefilm 205, refer to the description of the method for forming theconductive film which is to be the gate electrode 104.

Next, part of the conductive film 205 is removed so that the top surfaceof the conductive film 205 is level and part of the top surface of theinsulating film 249 is exposed. After that, parts of the insulating film249 and the conductive film 205 are removed so that the top surfaces ofthe insulating film 249 and the conductive film 205 are substantiallyflush with each other; thus, the gate electrode 204 and the insulatingfilm 218 whose top surface is substantially flush with that of the gateelectrode 204 are formed (see FIG. 7A). Note that with such removal, aneffect of unevenness due to the substrate 200 and other layers can bereduced, which enables the yield of the transistor to be improved.

Through the above steps, the transistor illustrated in FIGS. 4A to 4Ccan be manufactured.

Regions which function as the source region and the drain region of thetransistor have low resistance. Thus, a transistor including the oxidesemiconductor film has low parasitic resistance and has excellenton-state characteristics. Further, sources of generating carriers andthe like in the channel region of the oxide semiconductor film areextremely reduced. Thus, a transistor including such an oxidesemiconductor film has low off-state current and stable electriccharacteristics.

An insulating film and a wiring over the insulating film may be formedsubsequently. An example of a method for forming the wiring is describedbelow. First, an insulating film 220 is formed. Next, the gateinsulating film 212, the insulating film 218, and the insulating film220 are processed to expose the low-resistance region 206 a and thelow-resistance region 206 b which are included in the oxidesemiconductor film 206. Then, a conductive film which is to be a wiring224 a and a wiring 224 b is formed and is processed to form the wiring224 a and the wiring 224 b (see FIG. 7B).

For detailed method for forming the insulating film and the wiring overthe insulating film, refer to the description of the method for forminganother insulating film, another conductive film, or the like.

This embodiment can be implemented in appropriate combination with anyof the other embodiments and examples.

(Embodiment 3)

In this embodiment, a transistor having a structure different from thestructures of the transistors in Embodiment 1 and Embodiment 2 and amethod of manufacturing the transistor will be described.

FIG. 8A is a top view of a transistor of one embodiment of the presentinvention. FIG. 8B is a cross-sectional view along dashed-dotted lineC1-C2 of FIG. 8A. FIG. 8C is a cross-sectional view along dashed-dottedline C3-C4 of FIG. 8A. Note that a gate insulating film 312 and the likeare not illustrated in FIG. 8A for simplicity.

The transistor illustrated in FIG. 8B includes a base insulating film302 over a substrate 300; a gate electrode 304 over the base insulatingfilm 302; the gate insulating film 312 over the gate electrode 304; anoxide semiconductor film 306 which includes a low-resistance region 306a, a low-resistance region 306 b, and a high-resistance region 306 c andis over the gate electrode 304 with the gate insulating film 312provided therebetween; and a source electrode 316 a and a drainelectrode 316 b in contact with the low-resistance region 306 a and thelow-resistance region 306 b, respectively. Although the transistorillustrated in FIG. 8B includes the base insulating film 302, thepresent invention is not limited thereto. For example, the baseinsulating film 302 is not necessarily provided. Note that the sourceelectrode 316 a and the drain electrode 316 b may be collectivelyreferred to as a pair of electrodes.

Specifically, the low-resistance region 306 a and the low-resistanceregion 306 b included in the oxide semiconductor film 306 overlap withthe source electrode 316 a and the drain electrode 316 b, respectively.The high-resistance region 306 c included in the oxide semiconductorfilm 306 overlaps with neither the source electrode 316 a nor the drainelectrode 316 b. Thus, the low-resistance region 306 a and thelow-resistance region 306 b of the oxide semiconductor film 306 functionas a source region and a drain region of the transistor, respectively.Further, part of the high-resistance region 306 c of the oxidesemiconductor film 306 (a region between the source electrode 316 a andthe drain electrode 316 b) functions as a channel region of thetransistor.

For the oxide semiconductor film 306, refer to the description of theoxide semiconductor film 106. Further, for the low-resistance region 306a, the low-resistance region 306 b, and the high-resistance region 306 cincluded in the oxide semiconductor film 306, refer to the descriptionsof the low-resistance region 106 a, the low-resistance region 106 b, andthe high-resistance region 106 c included in the oxide semiconductorfilm 106.

For the substrate 300, refer to the description of the substrate 100.

For the base insulating film 302, refer to the description of the baseinsulating film 102.

For the gate electrode 304, refer to the description of the gateelectrode 104.

For the gate insulating film 312, refer to the description of the gateinsulating film 112.

For the source electrode 316 a and the drain electrode 316 b, refer tothe description of the source electrode 116 a and the drain electrode116 b.

Although not illustrated in FIGS. 8A to 8C, a protective insulating filmmay be provided over the oxide semiconductor film 306, the sourceelectrode 316 a, and the drain electrode 316 b.

As the protective insulating film, a single layer of an insulating filmsuch as a silicon oxide film, a silicon oxynitride film, a siliconnitride oxide film, a silicon nitride film, an aluminum oxide film, ahafnium oxide film, an yttrium oxide film, a zirconium oxide film, agallium oxide film, a tantalum oxide film, a magnesium oxide film, alanthanum oxide film, a cerium oxide film, or a neodymium oxide film ora stack of any of these insulating films is used. It is preferable touse a single layer of an insulating film such as a silicon oxide film, asilicon oxynitride film, a silicon nitride oxide film, or a siliconnitride film or a stack of any of these insulating films.

Note that the protective insulating film preferably contains excessoxygen. In the case where the protective insulating film contains excessoxygen, oxygen vacancies in the high-resistance region 306 c included inthe oxide semiconductor film 306 can be reduced.

A back gate electrode may be provided over the oxide semiconductor film306 with the protective insulating film provided therebetween. Thethreshold voltage can be controlled easily owing to the back gateelectrode. Moreover, the on-state current of the transistor can beincreased by electrically connecting the back gate electrode to the gateelectrode 304. Alternatively, the off-state current of the transistorcan be reduced by setting the potential of the back gate electrode toGND or a negative potential (a potential which is lower than that of thesource of the transistor or lower than GND).

Next, a method for manufacturing the transistor illustrated in FIGS. 8Ato 8C will be described. Note that the method for manufacturing thetransistor will be described with reference to cross-sectional viewseach corresponding to FIG. 8B.

First, the base insulating film 302 is formed over the substrate 300(see FIG. 9A). For the base insulating film 302, refer to thedescription of the method for forming the base insulating film 102.

Next, a conductive film to be the gate electrode 304 is formed. For theconductive film, refer to the description of the method for forming theconductive film which is to be the gate electrode 104.

After that, the conductive film to be the gate electrode 304 isprocessed to form the gate electrode 304 (see FIG. 9B).

Then, the gate insulating film 312 is formed (see FIG. 9C). For the gateinsulating film 312, refer to the description of the method for formingthe gate insulating film 112.

Next, an oxide semiconductor film 336 a is formed (see FIG. 9D). For theoxide semiconductor film 336 a, refer to the description of the methodfor forming the oxide semiconductor film 136 a.

Subsequently, process for reducing the resistance of the oxidesemiconductor film 336 a is performed to form an oxide semiconductorfilm 336 b having low resistance (see FIG. 10A). For the process forreducing the resistance of the oxide semiconductor film 336 a, refer tothe description in Embodiment 1.

Next, the oxide semiconductor film 336 b is processed into anisland-shaped oxide semiconductor film 336.

Then, a conductive film to be the source electrode 316 a and the drainelectrode 316 b is formed. For the conductive film, refer to thedescription of the conductive film to be the source electrode 116 a andthe drain electrode 116 b.

Then, a conductive film to be the source electrode 316 a and the drainelectrode 316 b is processed to form the source electrode 316 a and thedrain electrode 316 b (see FIG. 10B).

After that, a region of the oxide semiconductor film 336, which overlapswith neither the source electrode 316 a nor the drain electrode 316 b,is subjected to a process for increasing the resistance of the region ofthe oxide semiconductor film.

For a process for increasing the resistance of the oxide semiconductorfilm 336, refer to the description in Embodiment 1. In this embodiment,oxygen ions 330 are implanted (see FIG. 10C).

The oxide semiconductor film 336 having low resistance has low impurityconcentration and has many oxygen vacancies. Accordingly, the resistanceof the oxide semiconductor film having low resistance can be increasedby plasma oxidation or implantation of oxygen ions for reducing oxygenvacancies. An oxide semiconductor film having high resistance which isobtained in such a manner is an oxide semiconductor film having lowimpurity concentration and few oxygen vacancies.

The process for increasing the resistance of the oxide semiconductorfilm 336 enables the region of the oxide semiconductor film 336, whichoverlaps with neither the source electrode 316 a nor the drain electrode316 b, to be a high-resistance region. Meanwhile, regions of the oxidesemiconductor film 336, which overlap with the source electrode 316 aand the drain electrode 316 b, are low-resistance regions. In thismanner, the oxide semiconductor film 306 including the low-resistanceregion 306 a, the low-resistance region 306 b, and the high-resistanceregion 306 c can be formed (see FIG. 10D).

Note that the process for increasing the resistance of the oxidesemiconductor film 336 might oxidize surfaces of the source electrode316 a and the drain electrode 316 b. An increase in parasitic resistancedue to the oxidation of the surfaces of the source electrode 316 a andthe drain electrode 316 b can be ignored when the source electrode 316 aand the drain electrode 316 b have sufficient thicknesses. Specifically,the thicknesses of the source electrode 316 a and the drain electrode316 b are greater than or equal to 50 nm, preferably greater than orequal to 100 nm.

A region of the gate insulating film 312, which overlaps with thehigh-resistance region 306 c included in the oxide semiconductor film306, may contain excess oxygen by the process for increasing theresistance of the oxide semiconductor film 336 or by a similar process.

Through the above steps, the transistor illustrated in FIGS. 8A to 8Ccan be manufactured.

Regions which function as the source region and the drain region of thetransistor have low resistance. Thus, a transistor including the oxidesemiconductor film has low parasitic resistance and has excellenton-state characteristics. Further, sources of generating carriers andthe like in the channel region of the oxide semiconductor film areextremely reduced. Thus, a transistor including such an oxidesemiconductor film has low off-state current and stable electriccharacteristics.

This embodiment can be implemented in appropriate combination with anyof the other embodiments and examples.

(Embodiment 4)

In this embodiment, a logic circuit which is a semiconductor deviceaccording to one embodiment of the present invention is described.

FIG. 11A is a circuit diagram illustrating an example of a NOT circuit(inverter) using a p-channel transistor and an n-channel transistor.

A transistor Tr1 a which is a p-channel transistor may be, for example,a transistor including silicon. Note that the transistor Tr1 a is notlimited to a transistor including silicon. The threshold voltage of thetransistor Tr1 a is denoted by Vth1 a.

A transistor Tr2 a which is an n-channel transistor may be, for example,the transistor including an oxide semiconductor film, which is describedin the above embodiment. The threshold voltage of the transistor Tr2 ais denoted by Vth2 a.

Here, a gate of the transistor Tr1 a is connected to an input terminalVin and a gate of the transistor Tr2 a. A source of the transistor Tr1 ais electrically connected to a power supply potential (VDD). A drain ofthe transistor Tr1 a is connected to a drain of the transistor Tr2 a andan output terminal Vout. A source of the transistor Tr2 a is connectedto a ground potential (GND). A back gate of the transistor Tr2 a isconnected to a back gate line BGL. In this embodiment, the transistorTr2 a has a back gate; however, one embodiment of the present inventionis not limited thereto. For example, it is also possible to employ astructure in which the transistor Tr2 a does not have a back gate or astructure in which the transistor Tr1 a has a back gate.

For example, the threshold voltage Vth1 a of the transistor Tr1 a ishigher than VDD with an inverted sign and lower than 0 V (−VDD<Vth1 a<0V). Further, the threshold voltage Vth2 a of the transistor Tr2 a ishigher than 0 V and lower than VDD (0 V<Vth2 a<VDD). Note that a backgate may be used for control of the threshold voltage of eachtransistor.

Here, when the potential of the input terminal Vin is set to VDD, thegate voltage of the transistor Tr1 a becomes 0 V, so that the transistorTr1 a is turned off. Further, the gate voltage of the transistor Tr2 abecomes VDD, so that the transistor Tr2 a is turned on. Accordingly, theoutput terminal Vout is electrically connected to GND and supplied withGND.

When the potential of the input terminal Vin is set to GND, the gatevoltage of the transistor Tr1 a becomes VDD, so that the transistor Tr1a is turned on. Further, the gate voltage of the transistor Tr2 abecomes 0 V, so that the transistor Tr2 a is turned off. Accordingly,the output terminal Vout is electrically connected to VDD and suppliedwith VDD.

As described above, in the circuit diagram of FIG. 11A, GND is outputfrom the output terminal Vout when the potential of the input terminalVin is VDD, and VDD is output from the output terminal Vout when thepotential of the input terminal Vin is GND.

FIG. 11B is an example of a cross-sectional view of a semiconductordevice corresponding to FIG. 11A.

FIG. 11B is a cross-sectional view of the semiconductor device includingthe transistor Tr1 a over the transistor Tr1 a, and the transistor Tr2 aover the transistor Tr1 a.

In FIG. 11B, a transistor similar to the transistor illustrated in FIGS.1A to 1C is used as the transistor Tr2 a. Therefore, for components ofthe transistor Tr2 a which are not particularly described below, referto the description on FIGS. 1A to 1C.

Here, the transistor Tr1 a includes a semiconductor substrate 650, achannel region 656, a source region 657 a, and a drain region 657 bwhich are provided in the semiconductor substrate 650, an elementisolation layer 664 which fills a groove portion provided in thesemiconductor substrate 650, a gate insulating film 662 provided overthe semiconductor substrate 650, and a gate electrode 654 provided overthe channel region 656 with the gate insulating film 662 therebetween.

A single crystal semiconductor substrate or a polycrystallinesemiconductor substrate of silicon, silicon carbide, or the like or acompound semiconductor substrate of silicon germanium or the like may beused as the semiconductor substrate 650.

In this embodiment, the transistor Tr1 a is provided in a semiconductorsubstrate; however, one embodiment of the present invention is notlimited thereto. For example, a structure may be employed in which asubstrate having an insulating surface is used instead of thesemiconductor substrate and a semiconductor film is provided on theinsulating surface. Here, a glass substrate, a ceramic substrate, aquartz substrate, or a sapphire substrate may be used as the substratehaving an insulating surface, for example.

The source region 657 a and the drain region 657 b include an impuritywhich makes the semiconductor substrate 650 have p-type conductivity.

The element isolation layer 664 may be formed of a single layer or astacked layer using an insulating film containing one or more ofaluminum oxide, magnesium oxide, silicon oxide, silicon oxynitride,silicon nitride oxide, silicon nitride, gallium oxide, germanium oxide,yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide,hafnium oxide, and tantalum oxide.

The gate insulating film 662 may be formed of a single layer or astacked layer using an insulating film containing one or more ofaluminum oxide, magnesium oxide, silicon oxide, silicon oxynitride,silicon nitride oxide, silicon nitride, gallium oxide, germanium oxide,yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide,hafnium oxide, and tantalum oxide.

The gate electrode 654 may be formed of a conductive film of a singlelayer or a stacked layer of a simple substance selected from aluminum,titanium, chromium, cobalt, nickel, copper, yttrium, zirconium,molybdenum, silver, tantalum, and tungsten; a nitride containing one ormore kinds of the above substances; an oxide containing one or morekinds of the above substances; or an alloy containing one or more kindsof the above substances.

The gate electrode 654 functions not only as the gate electrode of thetransistor Tr1 a, but also as the gate electrode of the transistor Tr2a. Accordingly, the base insulating film 102 functions as the gateinsulating film of the transistor Tr2 a.

For a gate electrode 104 of the transistor Tr2 a, refer to thedescription of the gate electrode 104 of the transistor illustrated inFIGS. 1A to 1C. Note that the gate electrode 104 functions as a backgate electrode of the transistor Tr2 a.

In the semiconductor device illustrated in FIG. 11B, an insulating film690 whose top surface is aligned with the top surface of the gateelectrode 654 is provided. Note that a structure without the insulatingfilm 690 may be employed.

The insulating film 690 may be formed of a single layer or a stackedlayer using an insulating film containing one or more of aluminum oxide,magnesium oxide, silicon oxide, silicon oxynitride, silicon nitrideoxide, silicon nitride, gallium oxide, germanium oxide, yttrium oxide,zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, andtantalum oxide.

An opening reaching the drain region 657 b of the transistor Tr1 a isprovided in the gate insulating film 662, the insulating film 690, andthe base insulating film 102. The drain electrode 116 b of thetransistor Tr2 a is in contact with the drain region 657 b of thetransistor Tr1 a through the opening.

By applying the transistor described in the above embodiment to thetransistor Tr2 a, a flow-through current when the transistor Tr2 a isoff can be significantly reduced because the transistor Tr2 a has anextremely low off-state current. Thus, an inverter with low powerconsumption is achieved.

Note that the inverters illustrated in FIG. 11A may be combined to forma NAND circuit illustrated in FIG. 12A. The circuit diagram of FIG. 12Aincludes a transistor Tr1 b and a transistor Tr4 b which are p-channeltransistors, and a transistor Tr2 b and a transistor Tr3 b which aren-channel transistors. The transistors Tr1 b and Tr4 b may each be, forexample, a transistor including silicon. The transistors Tr2 b and Tr3 bmay each be the transistor including an oxide semiconductor filmdescribed in the above embodiment.

The inverter illustrated in FIG. 11A may be combined to form a NORcircuit illustrated in FIG. 12B. The circuit diagram of FIG. 12Bincludes a transistor Tr1 c and a transistor Tr2 c which are p-channeltransistors, and a transistor Tr3 c and a transistor Tr4 c which aren-channel transistors. The transistors Tr1 c and Tr2 c may each be, forexample, a transistor including silicon. The transistors Tr3 c and Tr4 cmay each be the transistor including an oxide semiconductor filmdescribed in the above embodiment.

The aforementioned examples of logic circuits are configured with theinverters using p-channel transistors and n-channel transistors; a logiccircuit may be configured with an inverter using only n-channeltransistors. FIG. 13A illustrates an example of such a case.

The circuit diagram of FIG. 13A includes a transistor Tr1 d which is adepletion transistor and a transistor Tr2 d which is an enhancementtransistor.

The depletion transistor Tr1 d may be, for example, a transistorincluding an oxide semiconductor film. Note that the transistor Tr1 d isnot limited to a transistor including an oxide semiconductor film, andmay be, for example, a transistor including silicon. The thresholdvoltage of the transistor Tr1 d is denoted by Vth1 d. A resistor with asufficiently low resistance may be provided instead of the depletiontransistor.

The enhancement transistor Tr2 d may be, for example, the transistorincluding an oxide semiconductor film described in the above embodiment.The threshold voltage of the transistor Tr2 d is denoted by Vth2 d.

Note that the transistor including an oxide semiconductor film describedin the above embodiment may be used as the transistor Tr1 d. In thatcase, a transistor other than the transistor including an oxidesemiconductor film described in the above embodiment may be used as thetransistor Tr2 d.

Here, a gate of the transistor Tr1 d is connected to an input terminalVin and a gate of the transistor Tr2 d. A drain of the transistor Tr1 dis electrically connected to VDD. A source of the transistor Tr1 d isconnected to a drain of the transistor Tr2 d and an output terminalVout. A source of the transistor Tr2 d is connected to GND. A back gateof the transistor Tr2 d is connected to a back gate line BGL. In thisembodiment, the transistor Tr2 d has a back gate; however, oneembodiment of the present invention is not limited thereto. For example,it is also possible to employ a structure in which the transistor Tr2 ddoes not have a back gate or a structure in which the transistor Tr1 dhas a back gate.

The threshold voltage Vth1 d of the transistor Tr1 d is, for example,lower than 0 V (Vth1 d<0 V). Accordingly, the transistor Tr1 d is onregardless of the gate voltage, that is, the transistor Tr1 d functionsas a resistor having a sufficiently low resistance. Further, thethreshold voltage Vth2 d of the transistor Tr2 d is higher than 0 V andlower than VDD (0 V<Vth2 d<VDD). Note that a back gate may be used forcontrol of the threshold voltage of each transistor. Further, a resistorhaving a sufficiently low resistance may be provided instead of thetransistor Tr1 d.

Here, when the potential of the input terminal Vin is set to VDD, thegate voltage of the transistor Tr2 d becomes VDD, so that the transistorTr2 d is turned on. Accordingly, the output terminal Vout iselectrically connected to GND and supplied with GND.

Further, when the potential of the input terminal Vin is set to GND, thegate voltage of the transistor Tr2 d becomes 0 V, so that the transistorTr2 d is turned off. Accordingly, the output terminal Vout iselectrically connected to VDD and supplied with VDD. Note that strictly,the potential output from the output terminal Vout is equal to apotential dropped from VDD by the resistance of the transistor Tr1 d.However, the effect of the voltage drop can be ignored because theresistance of the transistor Tr1 d is sufficiently low.

As described above, in the circuit diagram of FIG. 13A, when thepotential of the input terminal Vin is VDD, GND is output from theoutput terminal Vout, and when the potential of the input terminal Vinis GND, VDD is output from the output terminal Vout.

Note that the transistor Tr1 d and the transistor Tr2 d may bemanufactured in the same plane, which facilitates the production of theinverter. At this time, a back gate is preferably provided in at leastone of the transistors Tr1 d and Tr2 d. In the case where themanufactured transistors are depletion transistors, the thresholdvoltage Vth2 d may be set within the above range by the back gate of thetransistor Tr2 d. In the case where the manufactured transistors areenhancement transistors, the threshold voltage Vth1 d may be set withinthe above range by the back gate of the transistor Tr1 d. Note that thethreshold voltages of the transistors Tr1 d and Tr2 d may be controlledby different back gates.

Alternatively, the transistor Tr1 d and the transistor Tr2 d may overlapwith each other, in which case the inverter can be reduced in area.

FIG. 13B is an example of a cross-sectional view of a semiconductordevice in which the transistor Tr1 d and the transistor Tr2 d overlapwith each other.

For the transistor Tr1 d in FIG. 13B, refer to the description of thetransistor illustrated in FIGS. 1A to 1C. Further the transistor Tr2 dis different from the transistor illustrated in FIGS. 1A to 1C in thatpositions where the oxide semiconductor film is in contact with thesource electrode and the drain electrode. Thus, for components of thetransistor Tr2 d which are not particularly described below, refer tothe description on FIGS. 1A to 1C.

Note that the transistor Tr2 d includes an insulating film 802 over thetransistor Tr1 d; a source electrode 816 a and a drain electrode 816 bover the insulating film 802; an oxide semiconductor film 806 over theinsulating film 802, the source electrode 816 a, and the drain electrode816 b; a gate insulating film 812 over the oxide semiconductor film 806;and a gate electrode 814 which is over the gate insulating film 812 andoverlaps with the oxide semiconductor film 806.

The gate electrode 104 functions not only as the gate electrode of thetransistor Tr1 d, but also as the gate electrode of the transistor Tr2d. Accordingly, the insulating film 802 functions as the gate insulatingfilm of the transistor Tr2 d.

For the source electrode 816 a and the drain electrode 816 b of thetransistor Tr2 d, refer to the description of the source electrode 116 aand the drain electrode 116 b of the transistor illustrated in FIGS. 1Ato 1C.

For the oxide semiconductor film 806 of the transistor Tr2 d, refer tothe description of the oxide semiconductor film 106 of the transistorillustrated in FIGS. 1A to 1C.

For the gate insulating film 812 of the transistor Tr2 d, refer to thedescription of the gate insulating film 112 of the transistorillustrated in FIGS. 1A to 1C.

For a gate electrode 814 of the transistor Tr2 d, refer to thedescription of the gate electrode 104 of the transistor illustrated inFIGS. 1A to 1C. Note that the gate electrode 814 functions as a backgate electrode of the transistor Tr2 d.

In the semiconductor device illustrated in FIG. 13B, the insulating film120 whose top surface is aligned with the top surface of the gateelectrode 104 is provided. Note that a structure without the insulatingfilm 120 may be employed.

The insulating film 120 may be formed of a single layer or a stackedlayer using an insulating film containing one or more of aluminum oxide,magnesium oxide, silicon oxide, silicon oxynitride, silicon nitrideoxide, silicon nitride, gallium oxide, germanium oxide, yttrium oxide,zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, andtantalum oxide.

An opening reaching the drain electrode 116 b of the transistor Tr1 d isformed in the gate insulating film 112, the insulating film 120, and theinsulating film 802. The source electrode 816 a of the transistor Tr2 dis in contact with the drain electrode 116 b of the transistor Tr1 dthrough the opening.

By applying the transistor described in the above embodiment to thetransistor Tr1 d, a flow-through current when the transistor Tr1 d isoff can be significantly reduced because the transistor Tr1 d has anextremely low off-state current. Thus, an inverter with low powerconsumption is achieved.

This embodiment can be implemented in appropriate combination with anyof the other embodiments and examples.

(Embodiment 5)

In this embodiment, a static random access memory (SRAM) will bedescribed, the SRAM being a semiconductor device including a flip-flopwhich is obtained by applying the inverter circuit shown in Embodiment4.

In the SRAM, data is retained using a flip-flop; therefore, unlike in adynamic random access memory (DRAM), refresh operation is not necessaryand thus data can be retained with less power. In addition, the SRAMdoes not use a capacitor and thus is suitable for application requiringhigh-speed operation.

FIG. 14 is a circuit diagram equivalent to a memory cell of an SRAM ofone embodiment of the present invention. Although only one memory cellis illustrated in FIG. 14, one embodiment of the present invention maybe applied to a memory cell array including a plurality of the memorycells.

The memory cell illustrated in FIG. 14 includes a transistor Tr1 e, atransistor Tr2 e, a transistor Tr3 e, a transistor Tr4 e, a transistorTr5 e, and a transistor Tr6 e. The transistors Tr1 e and Tr2 e arep-channel transistors, and the transistors Tr3 e and Tr4 e are n-channeltransistors. A gate of the transistor Tr1 e is electrically connected toa drain of the transistor Tr2 e, a gate of the transistor Tr3 e, a drainof the transistor Tr4 e, and one of a source and a drain of thetransistor Tr6 e. A source of the transistor Tr1 e is electricallyconnected to VDD. A drain of the transistor Tr1 e is electricallyconnected to a gate of the transistor Tr2 e, a drain of the transistorTr3 e, a gate of the transistor Tr4 e, and one of a source and a drainof the transistor Tr5 e. A source of the transistor Tr2 e iselectrically connected to VDD. A source of the transistor Tr3 e iselectrically connected to GND. A back gate of the transistor Tr3 e iselectrically connected to a back gate line BGL. A source of thetransistor Tr4 e is electrically connected to GND. A back gate of thetransistor Tr4 e is electrically connected to a back gate line BGL. Agate of the transistor Tr5 e is electrically connected to the word lineWL. The other of the source and the drain of the transistor Tr5 e iselectrically connected to a bit line BLB. A gate of the transistor Tr6 eis electrically connected to the word line WL. The other of the sourceand the drain of the transistor Tr6 e is electrically connected to a bitline BL.

Note that this embodiment shows an example where n-channel transistorsare used as the transistors Tr5 e and Tr6 e. However, the transistorsTr5 e and Tr6 e are not limited to n-channel transistors and may bep-channel transistors. In that case, writing, retaining, and readingmethods described below may be changed as appropriate.

A flip-flop is thus configured in such a manner that an inverterincluding the transistors Tr1 e and Tr3 e and an inverter including thetransistors Tr2 e and Tr4 e are connected in a ring.

The p-channel transistors may be, but are not limited to, transistorsincluding silicon for example. The n-channel transistors may each be thetransistor including an oxide semiconductor film described in the aboveembodiment.

In this embodiment, the transistors Tr3 e and Tr4 e may each be thetransistor including an oxide semiconductor film described in the aboveembodiment. With an extremely low off-state current, the transistor hasan extremely low flow-through current.

Note that instead of the p-channel transistors, n-channel transistorsmay be applied to the transistors Tr1 e and Tr2 e. In the case wheren-channel transistors are used as the transistors Tr1 e and Tr2 e,depletion transistors may be employed as described with reference toFIGS. 13A and 13B.

Writing, retaining, and reading operation of the memory cell illustratedin FIG. 14 will be described below.

In writing, first, potentials corresponding to data 0 or data 1 areapplied to the bit line BL and the bit line BLB.

For example, in the case where data 1 is to be written, the VDD isapplied to the bit line BL and the GND is applied to the bit line BLB.Then, a potential (VH) higher than or equal to the sum of the VDD andthe threshold voltage of the transistors Tr5 e and Tr6 e is applied tothe word line WL.

Next, the potential of the word line WL is set to be lower than thethreshold voltage of the transistors Tr5 e and Tr6 e, whereby the data 1written to the flip-flop is retained. In the case of the SRAM, a currentflowing in retaining data is only the leakage current of thetransistors. Here, any of the transistors including an oxidesemiconductor film described in the above embodiment, which has anextremely low off-state current, is applied to some of the transistorsin the SRAM, resulting in a reduction in stand-by power for retainingdata because leakage current due to the transistor is extremely low.

In reading, the VDD is applied to the bit line BL and the bit line BLBin advance. Then, the VH is applied to the word line WL, so that thepotential of the bit line BLB is discharged through the transistors Tr5e and Tr3 e to be equal to the GND while the potential of the bit lineBL is kept at VDD. The potential difference between the bit line BL andthe bit line BLB is amplified by a sense amplifier (not illustrated),whereby the retained data 1 can be read.

In the case where data 0 is to be written, the GND is applied to the bitline BL and the VDD is applied to the bit line BLB; then, the VH isapplied to the word line WL. Next, the potential of the word line WL isset to be lower than the threshold voltage of the transistors Tr5 e andTr6 e, whereby the data 0 written to the flip-flop is retained. Inreading, the VDD is applied to the bit line BL and the bit line BLB inadvance. Then, the VH is applied to the word line WL, so that thepotential of the bit line BL is discharged through the transistors Tr6 eand Tr4 e to be equal to the GND while the potential of the bit line BLBis kept at VDD. The potential difference between the bit line BL and thebit line BLB is amplified by the sense amplifier, whereby the retaineddata 0 can be read.

According to this embodiment, an SRAM with low stand-by power can beprovided.

This embodiment can be implemented in appropriate combination with anyof the other embodiments and examples.

(Embodiment 6)

The transistors including an oxide semiconductor film described in theabove embodiment can have extremely low off-state current. That is, thetransistor has electrical characteristics in which leakage of chargethrough the transistor is unlikely to occur.

A semiconductor device which includes a transistor having such electriccharacteristics will be described below. The semiconductor deviceincludes a memory element which is functionally superior to a knownmemory element.

First, the semiconductor device will be specifically described withreference to FIGS. 15A to 15D. FIG. 15A is a circuit diagram showing amemory cell array of the semiconductor device. FIG. 15B is a circuitdiagram of a memory cell. FIG. 15C illustrates an example of across-sectional structure corresponding to the memory cell in FIG. 15B.FIG. 15D is a graph showing the electric characteristics of the memorycell in FIG. 15B.

The memory cell array in FIG. 15A includes a plurality of memory cells556, a plurality of bit lines 553, a plurality of word lines 554, aplurality of capacitor lines 555, and a plurality of sense amplifiers558.

Note that the bit lines 553 and the word lines 554 are provided in agrid pattern, and the memory cell 556 is provided for each intersectionof the bit line 553 and the word line 554. The bit lines 553 areconnected to the respective sense amplifiers 558. The sense amplifiers558 have a function of reading the potentials of the bit lines 553 asdata.

As shown in FIG. 15B, the memory cell 556 includes a transistor 551 anda capacitor 552. A gate of the transistor 551 is electrically connectedto the word line 554. A source of the transistor 551 is electricallyconnected to the bit line 553. A drain of the transistor 551 iselectrically connected to one terminal of the capacitor 552. The otherterminal of the capacitor 552 is electrically connected to the capacitorline 555.

FIG. 15C illustrates an example of a cross-sectional structure of thememory cell. FIG. 15C is a cross-sectional view of the semiconductordevice including the transistor 551, the wirings 224 a and 224 bconnected to the transistor 551, an insulating film 520 over thetransistor 551 and the wirings 224 a and 224 b, and the capacitor 552over the insulating film 520.

Note that in FIG. 15C, the transistor illustrated in FIGS. 4A to 4C isused as the transistor 551. Therefore, for components of the transistor551 which are not particularly described below, refer to the descriptionin the above embodiment.

The insulating film 520 may be formed of a single layer or a stackedlayer using an insulating film containing one or more of aluminum oxide,magnesium oxide, silicon oxide, silicon oxynitride, silicon nitrideoxide, silicon nitride, gallium oxide, germanium oxide, yttrium oxide,zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, andtantalum oxide. Alternatively, a resin film of a polyimide resin, anacrylic resin, an epoxy resin, a silicone resin, or the like may be usedas the insulating film 520.

The capacitor 552 includes an electrode 526 in contact with the wiring224 b, an electrode 528 overlapping with the electrode 526, and aninsulating film 522 provided between the electrode 526 and the electrode528.

The electrode 526 may be formed of a single layer or a stacked layer ofa conductive film of a simple substance selected from aluminum,titanium, chromium, cobalt, nickel, copper, yttrium, zirconium,molybdenum, silver, tantalum, and tungsten; a nitride containing one ormore kinds of the above substances; an oxide containing one or morekinds of the above substances; or an alloy containing one or more kindsof the above substances.

The electrode 528 may be formed of a single layer or a stacked layer ofa conductive film of a simple substance selected from aluminum,titanium, chromium, cobalt, nickel, copper, yttrium, zirconium,molybdenum, silver, tantalum, and tungsten; a nitride containing one ormore kinds of the above substances; an oxide containing one or morekinds of the above substances; or an alloy containing one or more kindsof the above substances.

The insulating film 522 may be formed of a single layer or a stackedlayer using an insulating film containing one or more of aluminum oxide,magnesium oxide, silicon oxide, silicon oxynitride, silicon nitrideoxide, silicon nitride, gallium oxide, germanium oxide, yttrium oxide,zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, andtantalum oxide.

Note that although FIG. 15C shows an example where the transistor 551and the capacitor 552 are provided in different layers, one embodimentof the present invention is not limited to this structure. For example,the transistor 551 and the capacitor 552 may be provided in the sameplane. With such a structure, memory cells having similar structures canbe disposed so as to overlap with each other, in which case, a largenumber of memory cells can be integrated in an area for one memory cell.Accordingly, the degree of integration of the semiconductor device canbe improved. Note that in this specification, “A overlaps with B” meansthat A and B are provided such that at least part of A overlaps with atleast part of B.

Here, the wiring 224 a in FIG. 15C is electrically connected to the bitline 553 in FIG. 15B. The gate electrode 204 in FIG. 15C is electricallyconnected to the word line 554 in FIG. 15B. The electrode 528 in FIG.15C is electrically connected to the capacitor line 555 in FIG. 15B.

As shown in FIG. 15D, a voltage held in the capacitor 552 graduallydecreases with time due to leakage through the transistor 551. A voltageoriginally charged from V0 to V1 is decreased with time to VA that is alimit for reading out data 1. This period is called a holding periodT_1. In the case of a two-level memory cell, refresh operation needs tobe performed within the holding period T_1.

For example, in the case where the off-state current of the transistor551 is not sufficiently small, the holding period T_1 becomes shortbecause the voltage held in the capacitor 552 significantly changes withtime. Accordingly, refresh operation needs to be frequently performed.An increase in frequency of refresh operation increases powerconsumption of the semiconductor device.

Since the off-state current of the transistor 551 is extremely small inthis embodiment, the holding period T_1 can be made extremely long.Further, the frequency of refresh operation can be reduced; thus, powerconsumption can be reduced. For example, in the case where a memory cellis formed using the transistor 551 having an off-state current of1×10⁻²¹ A to 1×10⁻²⁵ A, data can be held for several days to severaldecades without supply of electric power.

As described above, according to one embodiment of the presentinvention, a semiconductor device with high degree of integration andlow power consumption can be provided.

Next, a semiconductor device having a structure different from that ofthe semiconductor device in FIGS. 15A to 15D is described with referenceto FIGS. 16A to 16C. FIG. 16A is a circuit diagram showing a memory celland wirings included in the semiconductor device. FIG. 16B is a graphshowing the electric characteristics of the memory cell in FIG. 16A.FIG. 16C is an example of a cross-sectional view corresponding to thememory cell in FIG. 16A.

As shown in FIG. 16A, the memory cell includes a transistor 671, atransistor 672, and a capacitor 673. Here, a gate of the transistor 671is electrically connected to a word line 676. A source of the transistor671 is electrically connected to a source line 674. A drain of thetransistor 671 is electrically connected to a gate of the transistor 672and one terminal of the capacitor 673. A portion where the drain of thetransistor 671 is electrically connected to the gate of the transistor672 and the one terminal of the capacitor 673 is referred to as a node679. A source of the transistor 672 is electrically connected to asource line 675. A drain of the transistor 672 is electrically connectedto a drain line 677. The other terminal of the capacitor 673 iselectrically connected to a capacitor line 678.

The semiconductor device illustrated in FIGS. 16A to 16C utilizesvariation in the apparent threshold voltage of the transistor 672, whichdepends on the potential of the node 679. For example, FIG. 16B shows arelation between a voltage V_(CL), of the capacitor line 678 and a draincurrent I_(d) _(—) 2 flowing through the transistor 672.

The potential of the node 679 can be controlled through the transistor671. For example, the potential of the source line 674 is set to a powersupply potential VDD. In this case, when the potential of the word line676 is set to be higher than or equal to a potential obtained by addingthe power supply potential VDD to the threshold voltage Vth of thetransistor 671, the potential of the node 679 can be HIGH. Further, whenthe potential of the word line 676 is set to be lower than or equal tothe threshold voltage Vth of the transistor 671, the potential of thenode 679 can be LOW.

Thus, the transistor 672 has electric characteristics shown with eithera V_(CL)−I_(d) _(—) 2 curve denoted as LOW or a V_(CL)−I_(d) _(—) 2curve denoted as HIGH. That is, when the potential of the node 679 isLOW, I_(d) _(—) 2 is small at a V_(CL) of 0 V; accordingly, data 0 isstored. Further, when the potential of the node 679 is HIGH, I_(d) _(—)2 is large at a V_(CL) of 0 V; accordingly, data 1 is stored. In thismanner, data can be stored.

FIG. 16C illustrates an example of a cross-sectional structure of thememory cell. FIG. 16C is a cross-sectional view of the semiconductordevice including the transistor 672, an insulating film 668 over thetransistor 672, the transistor 671 over the insulating film 668, aninsulating film 620 over the transistor 671, and the capacitor 673 overthe insulating film 620.

The insulating film 620 may be formed of a single layer or a stackedlayer using an insulating film containing one or more of aluminum oxide,magnesium oxide, silicon oxide, silicon oxynitride, silicon nitrideoxide, silicon nitride, gallium oxide, germanium oxide, yttrium oxide,zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, andtantalum oxide. Alternatively, a resin film of a polyimide resin, anacrylic resin, an epoxy resin, a silicone resin, or the like may be usedas the insulating film 620.

Note that in FIG. 16C, the transistor illustrated in FIGS. 1A to 1C isused as the transistor 671. Therefore, for components of the transistor671 which are not particularly described below, refer to the descriptionin the above embodiment.

The transistor including crystalline silicon has an advantage thaton-state characteristics can be improved more easily than a transistorincluding an oxide semiconductor film. Therefore, it can be said thatthe transistor including crystalline silicon is suitable for thetransistor 672 for which excellent on-state characteristics arerequired.

Here, the transistor 672 includes the channel region 656 and impurityregions 657 which are provided in the semiconductor substrate 650, theelement isolation layer 664 which fills a groove portion provided in thesemiconductor substrate 650, the gate insulating film 662 provided overthe semiconductor substrate 650, and the gate electrode 654 providedover the channel region 656 with the gate insulating film 662therebetween. For the transistor 672, refer to the description of thetransistor Tr1 a illustrated in FIG. 11B.

The gate insulating film 668 may be formed of a single layer or astacked layer using an insulating film containing one or more ofaluminum oxide, magnesium oxide, silicon oxide, silicon oxynitride,silicon nitride oxide, silicon nitride, germanium oxide, yttrium oxide,zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, andtantalum oxide.

The insulating film 668 and the base insulating film 102 include anopening reaching the gate electrode 654 of the transistor 672. The drainelectrode 116 b of the transistor 671 is in contact with the gateelectrode 654 of the transistor 672 through the opening.

The capacitor 673 includes an electrode 626 in contact with the drainelectrode 116 b, an electrode 628 overlapping with the electrode 626,and an insulating film 622 provided between the electrode 626 and theelectrode 628.

For the electrode 626, refer to the description of the electrode 526.

For the electrode 628, refer to the description of the electrode 528.

Here, the source electrode 116 a in FIG. 16C is electrically connectedto the source line 674 in FIG. 16A. The gate electrode 404 in FIG. 16Cis electrically connected to the word line 676 in FIG. 16A. Further, theelectrode 628 in FIG. 16C is electrically connected to the capacitorline 678 in FIG. 16A.

Note that although FIG. 16C shows an example where the transistor 671and the capacitor 673 are provided in different layers, one embodimentof the present invention is not limited to this structure. For example,the transistor 671 and the capacitor 673 may be provided in the sameplane. With such a structure, memory cells having similar structures canbe disposed so as to overlap with each other, in which case, a largenumber of memory cells can be integrated in an area for one memory cell.Accordingly, the degree of integration of the semiconductor device canbe improved.

Here, when any of the transistors including an oxide semiconductor filmdescribed in the above embodiment is used as the transistor 671, chargeaccumulated in the node 679 can be prevented from leaking through thetransistor 671 because the off-state current of the transistor isextremely small. Therefore, data can be held for a long period. Further,a voltage necessary for writing data does not need to be high ascompared to the case of a flash memory; thus, power consumption can bemade lower and operation speed can be made higher.

As described above, according to one embodiment of the presentinvention, a semiconductor device with high degree of integration andlow power consumption can be provided.

This embodiment can be implemented in appropriate combination with anyof the other embodiments and examples.

(Embodiment 7)

A central processing unit (CPU) can be formed with the use of any of thetransistors including an oxide semiconductor film described in the aboveembodiment or any of the semiconductor devices including a memoryelement described in the above embodiment for at least part of the CPU.

FIG. 17A is a block diagram illustrating a specific structure of a CPU.The CPU illustrated in FIG. 17A includes an arithmetic logic unit (ALU)1191, an ALU controller 1192, an instruction decoder 1193, an interruptcontroller 1194, a timing controller 1195, a register 1196, a registercontroller 1197, a bus interface (Bus I/F) 1198, a rewritable ROM 1199,and an ROM interface (ROM I/F) 1189 over a substrate 1190. Asemiconductor substrate, an SOI substrate, a glass substrate, or thelike is used as the substrate 1190. The ROM 1199 and the ROM interface1189 may be provided over a separate chip. Obviously, the CPUillustrated in FIG. 17A is just an example in which the configuration issimplified, and actual CPUs may have a variety of configurationsdepending on the application.

An instruction input to the CPU through the bus interface 1198 is inputto the instruction decoder 1193, decoded therein, and then input to theALU controller 1192, the interrupt controller 1194, the registercontroller 1197, and the timing controller 1195.

The ALU controller 1192, the interrupt controller 1194, the registercontroller 1197, and the timing controller 1195 conduct a variety ofcontrols in accordance with the decoded instruction. Specifically, theALU controller 1192 generates signals for controlling the operation ofthe ALU 1191. While the CPU is executing a program, the interruptcontroller 1194 processes an interrupt request from an externalinput/output device or a peripheral circuit on the basis of its priorityor a mask state. The register controller 1197 generates an address ofthe register 1196, and reads/writes data from/to the register 1196 inaccordance with the state of the CPU.

The timing controller 1195 generates signals for controlling operationtimings of the ALU 1191, the ALU controller 1192, the instructiondecoder 1193, the interrupt controller 1194, and the register controller1197. For example, the timing controller 1195 includes an internal clockgenerator for generating an internal clock signal CLK2 based on areference clock signal CLK1, and supplies the clock signal CLK2 to theabove circuits.

In the CPU illustrated in FIG. 17A, a memory element is provided in theregister 1196. For the register 1196, any of the semiconductor devicesincluding a memory element described in the above embodiment can beused.

In the CPU illustrated in FIG. 17A, the register controller 1197 selectsoperation of retaining data in the register 1196 in accordance with aninstruction from the ALU 1191. That is, the register controller 1197selects whether data is retained by a flip flop or a capacitor in thememory element included in the register 1196. When data is retained bythe flip flop, a power supply voltage is supplied to the memory elementin the register 1196. When data is retained by the capacitor, the datain the capacitor is rewritten, and supply of the power supply voltage tothe memory element in the register 1196 can be stopped.

A switching element provided between a memory element group and a nodeto which a power supply potential VDD or a power supply potential VSS issupplied, as illustrated in FIG. 17B or FIG. 17C, allows the powersupply to be stopped. Circuits illustrated in FIGS. 17B and 17C aredescribed below.

FIGS. 17B and 17C each illustrate an example of a structure includingany of the transistors including an oxide semiconductor film describedin the above embodiment as a switching element for controlling supply ofa power supply potential to a memory element.

The memory device illustrated in FIG. 17B includes a switching element1141 and a memory element group 1143 including a plurality of memoryelements 1142. Specifically, as each of the memory elements 1142, any ofthe semiconductor devices including a memory element described in theabove embodiment can be used. Each of the memory elements 1142 includedin the memory element group 1143 is supplied with the high-level powersupply potential VDD through the switching element 1141. Further, eachof the memory elements 1142 included in the memory element group 1143 issupplied with a potential of a signal IN and the low-level power supplypotential VSS.

In FIG. 17B, as the switching element 1141, any of the transistorsincluding an oxide semiconductor film described in the above embodimentis used. The transistors can have extremely low off-state current. Theswitching of the transistor is controlled by a signal SigA input to agate thereof.

Note that FIG. 17B illustrates the structure in which the switchingelement 1141 includes only one transistor; however, one embodiment ofthe present invention is not limited thereto. The switching element 1141may include a plurality of transistors. In the case where the switchingelement 1141 includes a plurality of transistors functioning asswitching elements, the plurality of transistors may be connected toeach other in parallel, in series, or in combination of parallelconnection and series connection.

FIG. 17C shows an example of a memory device in which each of the memoryelements 1142 included in the memory element group 1143 is supplied withthe low-level power supply potential VSS through the switching element1141. The supply of the low-level power supply potential VSS to each ofthe memory elements 1142 included in the memory element group 1143 canbe controlled by the switching element 1141.

When a switching element is provided between a memory element group anda node to which the power supply potential VDD or the power supplypotential VSS is supplied, data can be retained even in the case wherean operation of a CPU is temporarily stopped and the supply of the powersupply voltage is stopped; accordingly, power consumption can bereduced. For example, while a user of a personal computer does not inputdata to an input device such as a keyboard, the operation of the CPU canbe stopped, so that the power consumption can be reduced.

Although the CPU is given as an example here, the transistor and thesemiconductor device can also be applied to an LSI such as a digitalsignal processor (DSP), a custom LSI, or a field programmable gate array(FPGA).

This embodiment can be implemented in appropriate combination with anyof the other embodiments and examples.

(Embodiment 8)

In this embodiment, a display device to which any of the transistorsdescribed in the above embodiment is applied will be described.

As a display element provided in the display device, a liquid crystalelement (also referred to as a liquid crystal display element), alight-emitting element (also referred to as a light-emitting displayelement), or the like can be used. A light-emitting element includes, inits category, an element whose luminance is controlled by current orvoltage, and specifically an inorganic electroluminescent (EL) element,an organic EL element, and the like. Furthermore, a display medium whosecontrast is changed by an electric effect, such as electronic ink, canbe used as the display element. In this embodiment, a display deviceincluding an EL element and a display device including a liquid crystalelement will be described as examples of the display device.

Note that the display device in this embodiment includes in its categorya panel in which a display element is sealed, and a module in which anIC such as a controller is mounted on the panel.

Additionally, the display device in this embodiment refers to an imagedisplay device, a display device, or a light source (including alighting device). The display device includes any of the followingmodules in its category: a module provided with a connector such as anFPC or TCP; a module in which a printed wiring board is provided at theend of TCP; and a module in which an integrated circuit (IC) is mounteddirectly on a display element by a COG method.

FIG. 18A is an example of a circuit diagram of the display deviceincluding an EL element.

The display device illustrated in FIG. 18A includes a switching element743, a transistor 741, a capacitor 742, and a light-emitting element719.

A gate of the transistor 741 is electrically connected to one terminalof the switch element 743 and one terminal of the capacitor 742. Asource of the transistor 741 is electrically connected to one terminalof the light-emitting element 719. A drain of the transistor 741 iselectrically connected to the other terminal of the capacitor 742 and issupplied with a power supply potential VDD. The other terminal of theswitch element 743 is electrically connected to a signal line 744. Theother terminal of the light-emitting element 719 is supplied with afixed potential. Note that the fixed potential is a ground potential GNDor lower.

As the transistor 741, any of the transistors including an oxidesemiconductor film described in the above embodiment is used. Thetransistor has favorable switching characteristics. Consequently, adisplay device having high display quality can be obtained.

As the switching element 743, it is preferred to use a transistor. Witha transistor, the area of a pixel can be reduced, so that a displaydevice having a high resolution can be obtained. Moreover, as theswitching element 743, any of the transistors including an oxidesemiconductor film described in the above embodiment may be used. Withthe use of the transistor as the switching element 743, the switchingelement 743 can be formed in the same process as the transistor 741;thus, the productivity of the display device can be improved.

FIG. 18B illustrates part of a cross section of a pixel including thetransistor 741, the capacitor 742, and the light-emitting element 719.

Note that FIG. 18B shows an example where the transistor 741 and thecapacitor 742 are provided in the same plane. With such a structure, thecapacitor 742 can be formed in the same plane as a gate electrode, agate insulating film, and a source electrode (drain electrode), whichare included in the transistor 741. When the transistor 741 and thecapacitor 742 are provided in the same plane in this manner, the numberof manufacturing steps of the display device can be reduced; thus, theproductivity can be increased.

In FIG. 18B, the transistor 741 has a structure in which a protectiveinsulating film 318 is included in the transistor illustrated in FIGS.8A to 8C. Thus, for components of the transistor 741 which are notparticularly described below, refer to the description in the aboveembodiment. Note that for the protective insulating film 318, refer tothe description of the base insulating film 102.

An insulating film 720 is provided over the transistor 741 and thecapacitor 742.

Here, an opening reaching the source electrode 316 a of the transistor741 is provided in the insulating film 720 and the protective insulatingfilm 318.

An electrode 781 is provided over the insulating film 720. The electrode781 is connected to the source electrode 316 a of the transistor 741through the opening provided in the insulating film 720 and theprotective insulating film 318.

A partition 784 having an opening reaching the electrode 781 is providedover the electrode 781.

A light-emitting layer 782 in contact with the electrode 781 through theopening provided in the partition 784 is provided over the partition784.

An electrode 783 is provided over the light-emitting layer 782.

A region where the electrode 781, the light-emitting layer 782, and theelectrode 783 overlap with one another serves as the light-emittingelement 719.

The insulating film 720 may be formed of a single layer or a stackedlayer using an insulating film containing one or more of aluminum oxide,magnesium oxide, silicon oxide, silicon oxynitride, silicon nitrideoxide, silicon nitride, gallium oxide, germanium oxide, yttrium oxide,zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, andtantalum oxide. Alternatively, a resin film of a polyimide resin, anacrylic resin, an epoxy resin, a silicone resin, or the like may beused.

The light-emitting layer 782 is not limited to a single layer, and maybe a stack of plural kinds of light-emitting layers and the like. Forexample, a structure illustrated in FIG. 18C may be employed. FIG. 18Cillustrates the structure in which an intermediate layer 785 a, alight-emitting layer 786 a, an intermediate layer 785 b, alight-emitting layer 786 b, an intermediate layer 785 c, alight-emitting layer 786 c, and an intermediate layer 785 d are stackedin this order. In that case, when light-emitting layers emitting lightof appropriate colors are used as the light-emitting layer 786 a, thelight-emitting layer 786 b, and the light-emitting layer 786 c, thelight-emitting element 719 with a high color rendering property orhigher luminous efficiency can be formed.

White light may be obtained by stacking plural kinds of light-emittinglayers. Although not illustrated in FIG. 18B, white light may beextracted through coloring layers.

Although the structure in which three light-emitting layers and fourintermediate layers are provided is shown here, the number oflight-emitting layers and the number of intermediate layers can bechanged as appropriate. For example, the light-emitting layer 782 can beformed with only the intermediate layer 785 a, the light-emitting layer786 a, the intermediate layer 785 b, the light-emitting layer 786 b, andthe intermediate layer 785 c. Alternatively, the light-emitting layer782 may be formed with the intermediate layer 785 a, the light-emittinglayer 786 a, the intermediate layer 785 b, the light-emitting layer 786b, the light-emitting layer 786 c, and the intermediate layer 785 d; theintermediate layer 785 c may be omitted.

Further, the intermediate layer can be formed using a stacked-layerstructure including any of a hole-injection layer, a hole-transportlayer, an electron-transport layer, an electron-injection layer, and thelike. Note that not all of these layers need to be provided as theintermediate layer. Any of these layers may be selected as appropriateto form the intermediate layer. Note that layers having similarfunctions may be provided. Further, an electron-relay layer or the likemay be added as appropriate as the intermediate layer, in addition to acarrier generation layer.

The electrode 781 can be formed using a conductive film having atransmitting property with respect to visible light. To have atransmitting property with respect to visible light means that theaverage transmittance of light in a visible light region (for example, awavelength range from 400 nm to 800 nm) is higher than or equal to 70%,particularly higher than or equal to 80%.

As the electrode 781, for example, an oxide film such as an In—Zn—Woxide film, an In—Sn oxide film, an In—Zn oxide film, an In oxide film,a Zn oxide film, or a Sn oxide film may be used. The above oxide filmmay contain a minute amount of Al, Ga, Sb, F, or the like. Further, ametal thin film having a thickness enough to transmit light (preferably,approximately 5 nm to 30 nm) can also be used. For example, an Ag film,a Mg film, or an Ag—Mg alloy film with a thickness of 5 nm may be used.

The electrode 781 is preferred to be a film which efficiently reflectsvisible light. For example, a film containing lithium, aluminum,titanium, magnesium, lanthanum, silver, silicon, or nickel can be usedas the electrode 781.

The electrode 783 can be formed using any of the films for the electrode781. Note that when the electrode 781 has a transmitting property withrespect to visible light, it is preferred that the electrode 783efficiently reflect visible light. When the electrode 781 efficientlyreflects visible light, it is preferred that the electrode 783 have atransmitting property with respect to visible light.

Positions of the electrode 781 and the electrode 783 are not limited tothe structure illustrated in FIG. 18B, and the electrode 781 and theelectrode 783 may be replaced with each other. It is preferred to use aconductive film having a high work function for the electrode whichserves as an anode and a conductive film having a low work function forthe electrode which serves as a cathode. Note that in the case where acarrier generation layer is provided in contact with the anode, avariety of conductive films can be used for the anode regardless oftheir work functions.

The partition 784 may be formed of a single layer or a stacked layerusing an insulating film containing one or more of aluminum oxide,magnesium oxide, silicon oxide, silicon oxynitride, silicon nitrideoxide, silicon nitride, gallium oxide, germanium oxide, yttrium oxide,zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, andtantalum oxide. Alternatively, a resin film of a polyimide resin, anacrylic resin, an epoxy resin, a silicone resin, or the like may beused.

The transistor 741 connected to the light-emitting element 719 hasexcellent on-state characteristics and stable electric characteristics.Therefore, a display device having high display quality can be provided.

Next, the display device including a liquid crystal element isdescribed.

FIG. 19A is a circuit diagram illustrating a structure example of thepixel of the display device including a liquid crystal element. A pixel750 illustrated in FIG. 19A includes a transistor 751, a capacitor 752,and an element in which a space between a pair of electrodes is filledwith liquid crystal (hereinafter also referred to as a liquid crystalelement) 753.

One of a source and a drain of the transistor 751 is electricallyconnected to a signal line 755, and a gate of the transistor 751 iselectrically connected to a scan line 754.

One of electrodes of the capacitor 752 is electrically connected to theother of the source and the drain of the transistor 751, and the otherof the electrodes of the capacitor 752 is electrically connected to awiring for supplying a common potential.

One of electrodes of the liquid crystal element 753 is electricallyconnected to the other of the source and the drain of the transistor751, and the other of the electrodes of the liquid crystal element 753is electrically connected to a wiring for supplying a common potential.Note that the common potential supplied to the other of the electrodesof the liquid crystal element 753 may be different from the commonpotential supplied to the wiring electrically connected to the other ofthe electrodes of the capacitor 752.

FIG. 19B illustrates part of a cross section of the pixel 750.

Note that FIG. 19B shows an example where the transistor 751 and thecapacitor 752 are provided in the same plane. With such a structure, thecapacitor 752 can be formed in the same plane as a gate electrode, agate insulating film, and a source electrode (drain electrode), whichare included in the transistor 751. When the transistor 751 and thecapacitor 752 are provided in the same plane in this manner, the numberof manufacturing steps of the display device can be reduced; thus, theproductivity can be increased.

As the transistor 751, any of the transistors described in the aboveembodiment can be used. In FIG. 19B, the transistor illustrated in FIGS.8A to 8C is used as the transistor 751. Thus, for components of thetransistor 751 which are not particularly described below, refer to thedescription in the above embodiment. Note that the transistor 751 has astructure in which the protective insulating film 318 is added to thetransistor illustrated in FIGS. 8A to 8C.

Note that the transistor 751 can be a transistor having extremely lowoff-state current. Thus, charge held in the capacitor 752 is unlikely toleak and a voltage applied to the liquid crystal element 753 can beretained for a long time. Accordingly, when a motion image with lessmovement or a still image is displayed, power for operating thetransistor 751 is not needed by turning off the transistor 751, wherebya display device with low power consumption can be obtained.

An insulating film 721 is provided over the transistor 751 and thecapacitor 752.

Here, an opening reaching the drain electrode 316 b of the transistor751 is provided in the insulating film 721 and the protective insulatingfilm 318.

An electrode 791 is provided over the insulating film 721. The electrode791 is in contact with the drain electrode 316 b of the transistor 751through the opening provided in the insulating film 721 and theprotective insulating film 318.

An insulating film 792 serving as an alignment film is provided over theelectrode 791.

A liquid crystal layer 793 is provided over the insulating film 792.

An insulating film 794 serving as an alignment film is provided over theliquid crystal layer 793.

A spacer 795 is provided over the insulating film 794.

An electrode 796 is provided over the spacer 795 and the insulating film794.

A substrate 797 is provided over the electrode 796.

The insulating film 721 may be formed of a single layer or a stackedlayer using an insulating film containing one or more of aluminum oxide,magnesium oxide, silicon oxide, silicon oxynitride, silicon nitrideoxide, silicon nitride, gallium oxide, germanium oxide, yttrium oxide,zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, andtantalum oxide. Alternatively, a resin film of a polyimide resin, anacrylic resin, an epoxy resin, a silicone resin, or the like may beused.

For the liquid crystal layer 793, a thermotropic liquid crystal, alow-molecular liquid crystal, a polymer liquid crystal, apolymer-dispersed liquid crystal, a ferroelectric liquid crystal, ananti-ferroelectric liquid crystal, or the like can be used. Such aliquid crystal material exhibits a cholesteric phase, a smectic phase, acubic phase, a chiral nematic phase, an isotropic phase, or the likedepending on conditions.

For the liquid crystal layer 793, a liquid crystal exhibiting a bluephase may be used. In that case, the insulating films 792 and 794serving as an alignment film are not necessarily.

The electrode 791 can be formed using a conductive film having atransmitting property with respect to visible light.

As the electrode 791, for example, an oxide film such as an In—Zn—Woxide film, an In—Sn oxide film, an In—Zn oxide film, an In oxide film,a Zn oxide film, or a Sn oxide film may be used. The above oxide filmmay contain a minute amount of Al, Ga, Sb, F, or the like. Further, ametal thin film having a thickness enough to transmit light (preferably,approximately 5 nm to 30 nm) can also be used.

The electrode 791 is preferred to be a film which efficiently reflectsvisible light. For example, a film containing aluminum, titanium,chromium, copper, molybdenum, silver, tantalum, or tungsten can be usedas the electrode 791.

The electrode 796 can be formed using any of the films for the electrode791. Note that when the electrode 791 has a transmitting property withrespect to visible light, it is preferred that the electrode 796efficiently reflect visible light. When the electrode 791 efficientlyreflects visible light, it is preferred that the electrode 796 have atransmitting property with respect to visible light.

Positions of the electrode 791 and the electrode 796 are not limited tothe structure illustrated in FIG. 19B, and the electrode 791 and theelectrode 796 may be replaced with each other.

Each of the insulating films 792 and 794 may be formed using an organiccompound or an inorganic compound.

The spacer 795 may be formed using an organic compound or an inorganiccompound. Note that the spacer 795 can have a variety of shapes such asa columnar shape and a spherical shape.

A region where the electrode 791, the insulating film 792, the liquidcrystal layer 793, the insulating film 794, and the electrode 796overlap with one another serves as the liquid crystal element 753.

As the substrate 797, a glass substrate, a resin substrate, a metalsubstrate, or the like can be used. The substrate 797 may haveflexibility.

The transistor 751 connected to the liquid crystal element 753 hasexcellent on-state characteristics and stable electric characteristics.Therefore, a display device having high display quality can be provided.Since the transistor 751 can have extremely low off-state current, adisplay device with low power consumption can be provided.

This embodiment can be implemented in appropriate combination with anyof the other embodiments and examples.

(Embodiment 9)

In this embodiment, examples of an electronic device including any ofthe semiconductor devices described in the above embodiments will bedescribed.

FIG. 20A illustrates a portable information terminal. The portableinformation terminal illustrated in FIG. 20A includes a housing 9300, abutton 9301, a microphone 9302, a display portion 9303, a speaker 9304,and a camera 9305, and has a function as a mobile phone. One embodimentof the present invention can be applied to an arithmetic unit, awireless circuit, or a memory circuit in a main body. One embodiment ofthe present invention can also be applied to the display portion 9303.

FIG. 20B illustrates a display. The display illustrated in FIG. 20Bincludes a housing 9310 and a display portion 9311. One embodiment ofthe present invention can be applied to an arithmetic unit, a wirelesscircuit, or a memory circuit in a main body. One embodiment of thepresent invention can also be applied to the display portion 9311.

FIG. 20C illustrates a digital still camera. The digital still cameraillustrated in FIG. 20C includes a housing 9320, a button 9321, amicrophone 9322, and a display portion 9323. One embodiment of thepresent invention can be applied to an arithmetic unit, a wirelesscircuit, or a memory circuit in a main body. One embodiment of thepresent invention can also be applied to the display portion 9323.

FIG. 20D illustrates a double-foldable portable information terminal.The double-foldable portable information terminal illustrated in FIG.20D includes a housing 9630, a display portion 9631 a, a display portion9631 b, a hinge 9633, and an operation switch 9638. One embodiment ofthe present invention can be applied to an arithmetic unit, a wirelesscircuit, or a memory circuit in a main body. One embodiment of thepresent invention can also be applied to the display portion 9631 a andthe display portion 9631 b.

Part or whole of the display portion 9631 a and/or the display portion9631 b can function as a touch panel. By touching an operation keydisplayed on the touch panel, a user can input data, for example.

With the use of a semiconductor device according to one embodiment ofthe present invention, an electronic device with high performance andlow power consumption can be provided.

This embodiment can be implemented in appropriate combination with anyof the other embodiments and examples.

Example 1

This example describes an example in which a process for increasing theresistance of an oxide semiconductor film is performed.

A method for forming samples is described below.

First, a glass substrate was prepared.

Then a silicon oxide film was formed to a thickness of 300 nm.

Next, a CMP process was performed to reduce roughness of the top surfaceof the silicon oxide film and to planarize the surface of the siliconoxide film.

Subsequently, an aluminum oxide film was formed to a thickness of 10 nm.

After that, a first oxide semiconductor film was formed to a thicknessof 10 nm, 15 nm, or 30 nm. The first oxide semiconductor film was formedby a sputtering method using an In—Ga—Zn oxide (In:Ga:Zn=1:1:1 [atomicratio]) target under the following conditions: a flow rate of an oxygengas was 45 sccm, the pressure was controlled to 0.4 Pa, the distancebetween a substrate and the target was 60 mm, the substrate temperaturewas 300° C., and a power of 0.5 kW (DC) was applied.

Alternatively, a second oxide semiconductor film was formed to athickness of 5 nm, 10 nm, 15 nm, or 30 nm. The second oxidesemiconductor film was formed by a sputtering method using an In—Sn—Znoxide (In:Sn:Zn=1:1:1 [atomic ratio]) target under the followingconditions: a flow rate of an oxygen gas was 50 sccm, the pressure wascontrolled to 0.4 Pa, the distance between a substrate and the targetwas 60 mm, the substrate temperature was 200° C., and a power of 0.5 kW(DC) was applied.

Next, the formed samples were subjected to a laser process. The laserprocess was performed with an excimer laser which emits light having anemission wavelength of 308 nm, with an energy density greater than orequal to 120.9 mJ/cm² and less than or equal to 700.4 mJ/cm².

FIGS. 21A and 21B show relations between the energy density and thesheet resistance of the samples after the laser process. The sheetresistance was measured by a four-probe method. Note that in FIGS. 21Aand 21B, hollow circles are plots of the sample including a 5-nm-thickoxide semiconductor film (not found in FIG. 21A), solid circles areplots of the sample including a 10-nm-thick oxide semiconductor film,hollow triangles are plots of the sample including a 15-nm-thick oxidesemiconductor film, and solid triangles are plots of the sampleincluding a 30-nm-thick oxide semiconductor film. FIG. 21A shows therelations between the energy density and the sheet resistance of thesamples including the first oxide semiconductor film, and FIG. 21B showthe relations between the energy density and the sheet resistance of thesamples including the second oxide semiconductor film.

FIGS. 21A and 21B show that it is possible to reduce the sheetresistance of the oxide semiconductor film by adjusting the energydensity of the laser process.

Then, the crystal states of samples in which an oxide semiconductor filmwas subjected to a laser process were evaluated. The crystal state wasevaluated by an X-ray diffraction (XRD) and with a transmission electronmicroscope (TEM).

The XRD measurement was conducted using an X-ray diffractometer D8ADVANCE manufactured by Bruker AXS, and the measurement was performed byan out-of-plane method. Note that H-9000NAR manufactured by Hitachi, Ltdwas used as the TEM.

A method for forming samples is described below.

First, a glass substrate was prepared.

Then a silicon oxide film was formed to a thickness of 300 nm

Subsequently, an aluminum oxide film was formed to a thickness of 20 nm.

After that, an oxide semiconductor film was formed to a thickness of 30nm. The oxide semiconductor film was formed by a sputtering method usingan In—Ga—Zn oxide (In:Ga:Zn=1:1:1 [atomic ratio]) target under thefollowing conditions: a flow rate of an argon gas was 30 sccm and a flowrate of an oxygen gas was 15 sccm, the pressure was controlled to 0.4Pa, the distance between a substrate and the target was 60 mm, thesubstrate temperature was 300° C., and a power of 0.5 kW (DC) wasapplied.

Next, the formed samples were subjected to a laser process. The laserprocess was performed with an excimer laser which emits light having anemission wavelength of 308 nm, with an energy density of 140 mJ/cm², 201mJ/cm², 300 mJ/cm², 349 mJ/cm², or 451 mJ/cm².

Next, the samples were measured by XRD. The results are shown in FIG.22. Note that in FIG. 22, a curve 4001, a curve 4002, a curve 4003, acurve 4004, and a curve 4005 are for the samples obtained with energydensities of 140 mJ/cm², 201 mJ/cm², 300 mJ/cm², 349 mJ/cm², and 451mJ/cm², respectively.

In FIG. 22, no particular peak of diffraction intensity can be seen forthe samples obtained with energy densities of 140 mJ/cm² and 201 mJ/cm².Meanwhile, a peak of diffraction intensity of the (0 0 9) plane can beseen for the sample obtained with an energy density of 300 mJ/cm².Further, peaks of diffraction intensity of the (0 0 6) plane, the (0 09) plane, the (0 0 12) plane, the (0 0 15) plane, and the (0 0 18) planecan be seen for the sample obtained with an energy density of 349mJ/cm²; the peak of diffraction intensity of the (0 0 9) plane isparticularly high. In addition, peaks of diffraction intensity of the (00 6) plane and the (0 0 9) plane can be seen for the sample obtainedwith an energy density of 451 mJ/cm². Note that the peaks of diffractionintensity of the (0 0 6) plane, the (0 0 9) plane, the (0 0 12) plane,the (0 0 15) plane, and the (0 0 18) plane exist where 2θ=20.42°,2θ=30.84°, 2θ=41.52°, 2θ=52.62°, and 2θ=64.23°, respectively.

Then, transmitted electron (TE) images (also referred to as TEM images)of cross sections of the same samples were observed with a TEM. Notethat the acceleration voltage was 300 kV and the magnification was4000000 times. The results are shown in FIGS. 23A and 23B, FIGS. 24A and24B, and FIGS. 25A and 25B. FIG. 23A, FIG. 23B, FIG. 24A, FIG. 24B, andFIG. 25A show the results of the samples obtained with energy densitiesof 140 mJ/cm², 201 mJ/cm², 300 mJ/cm², 349 mJ/cm², and 451 mJ/cm²,respectively.

FIGS. 23A and 23B show that oxide semiconductor films in the samplesobtained with energy densities of 140 mJ/cm² and 201 mJ/cm² areamorphous. FIG. 24A shows that an oxide semiconductor film in the sampleobtained with an energy density of 300 mJ/cm² is crystallized. FIG. 24Bshows that an oxide semiconductor film in the sample obtained with anenergy density of 349 mJ/cm² is crystallized. This sample ispolycrystalline. FIGS. 25A and 25B show that an oxide semiconductor filmin the sample obtained with an energy density of 451 mJ/cm² iscrystallized. The crystallinity of this sample is lower than that of thesample obtained with an energy density of 349 mJ/cm². Note that FIG. 25Bis a TE image of the observation portion in FIG. 25A observed with amagnification of 2000000 times. As can be seen in FIG. 25B, a mixedlayer of a silicon oxynitride film, an aluminum oxide film, and an oxidesemiconductor film is formed in the sample obtained with an energydensity of 451 mJ/cm².

From the above, it is found that an increase in energy density of alaser process enables an oxide semiconductor film to be crystallized.However, it is also found that too high energy density of a laserprocess causes a reduction in crystallinity and formation of a mixedregion of the oxide semiconductor film and a base film. Accordingly, itis important that a laser process needs to be performed with anappropriate energy density to increase the crystallinity of an oxidesemiconductor film.

Comparisons of the sheet resistance and the crystal states of oxidesemiconductor films give that the sheet resistance of the oxidesemiconductor film is reduced because the oxide semiconductor film iscrystallized. The comparisons also give that too high energy density ofa laser process causes a reduction in crystallinity and formation of amixed region of the oxide semiconductor film and a base film, resultingin an increase in sheet resistance.

Example 2

This example describes an example in which a process for increasing theresistance of an oxide semiconductor film whose resistance is reduced bya laser process is performed.

A method for forming samples is described below.

First, a glass substrate was prepared.

Then a silicon oxide film was formed to a thickness of 300 nm.

Subsequently, an aluminum oxide film was formed to a thickness of 10 nm.

After that, a first oxide semiconductor film was formed to a thicknessof 30 nm. The first oxide semiconductor film was formed by a sputteringmethod using an In—Ga—Zn oxide (In:Ga:Zn=1:1:1 [atomic ratio]) targetunder the following conditions: a flow rate of an oxygen gas was 45sccm, the pressure was controlled to 0.4 Pa, the distance between asubstrate and the target was 60 mm, the substrate temperature was 300°C., and a power of 0.5 kW (DC) was applied.

Alternatively, a second oxide semiconductor film was formed to athickness of 15 nm. The second oxide semiconductor film was formed by asputtering method using an In—Sn—Zn oxide (In:Sn:Zn=2:1:3 [atomicratio]) target under the following conditions: a flow rate of an oxygengas was 50 sccm, the pressure was controlled to 0.4 Pa, the distancebetween a substrate and the target was 60 mm, the substrate temperaturewas 200° C., and a power of 0.5 kW (DC) was applied.

Next, the formed samples were subjected to a laser process. The laserprocess was performed with an excimer laser which emits light having anemission wavelength of 308 nm. The sample including the first oxidesemiconductor film was obtained with an energy density of 391 mJ/cm² andthe sample including the second oxide semiconductor film was obtainedwith an energy density of 298 mJ/cm².

Other formed samples were subjected to heat treatment. The samples wereheated at 450° C. in a nitrogen gas atmosphere for one hour and thenheated at 450° C. in an oxygen gas atmosphere for one hour.

Then, the oxide semiconductor film was processed to form anisland-shaped oxide semiconductor film.

After that, a silicon oxynitride film was formed to a thickness of 100nm.

Some of the samples were implanted with oxygen ions. The oxygen ionswere implanted at an acceleration voltage of 25 kV with a dosage of1×10¹⁶ ions/cm².

Next, an aluminum oxide film was formed to a thickness of 50 nm.

Subsequently, a silicon oxynitride film was formed to a thickness of 300nm.

Then, the 100-nm-thick silicon oxynitride film, the 50-nm-thick aluminumoxide film, and the 300-nm-thick silicon oxynitride film were processedso that part of the oxide semiconductor film was exposed.

After that, a titanium film was formed to a thickness of 300 nm.

Next, the titanium film was processed so that a pair of electrodes atleast partly in contact with the oxide semiconductor film was formed.

Subsequently, the formed samples were subjected to heat treatment. Thesamples were heated at 300° C. in an air atmosphere for one hour.

Then, the resistance between the electrodes formed using a titanium filmwas measured. Note that the electrodes were provided apart from eachother. Since the pair of electrodes was formed using a titanium film,the resistance thereof was sufficiently lower than that of the oxidesemiconductor film; thus, the resistance between the electrodes wasalmost the same as that of the oxide semiconductor film.

A region of the oxide semiconductor film which was between theelectrodes had a width of 69100 μm and the distance between theelectrodes of 3 μm, 10 μm, 50 μm, 100 μm, 200 μm, or 500 μm. Then, theresistance was measured at six points in one sample. Next, the distancebetween the electrodes and the resistance measured at six points wereplotted on the lateral axis and the vertical axis, respectively. Afterthat, the slope of a straight line obtained by linear approximation ofthe plots was multiplied by the electrode width (69100 μm) to obtain thesheet resistance.

FIG. 26 shows the sheet resistance of each sample and compares thesamples with and without oxygen ion implantation. In FIG. 26, a hollowbar represents the sheet resistance of the sample including the firstoxide semiconductor film, and a shaded bar represents the sheetresistance of the sample including the second oxide semiconductor film.

FIG. 26 shows that the resistivity of the oxide semiconductor film whichis reduced by the laser process increases by the implantation of oxygenions in all the samples.

This application is based on Japanese Patent Application serial no.2012-123951 filed with Japan Patent Office on May 31, 2012, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A method for manufacturing a semiconductordevice, comprising the steps of: forming an oxide semiconductor layerover an insulating surface; performing a first treatment so that aresistance of the oxide semiconductor layer is reduced; forming a firstinsulating layer over the oxide semiconductor layer after performing thefirst treatment; forming a sacrificial layer overlapping with a part ofthe oxide semiconductor layer with the first insulating layer interposedtherebetween; forming a second insulating layer over the sacrificiallayer; removing a part of the second insulating layer and a part of thesacrificial layer so that a top surface of the sacrificial layer isexposed and the top surface of the sacrificial layer is substantiallyflush with a top surface of the rest of the second insulating layer;removing the sacrificial layer, thereby forming a third insulating layerwhich does not cover a part of the first insulating layer; performing asecond treatment after removing the sacrificial layer; forming aconductive layer over the first insulating layer and the thirdinsulating layer after performing the second treatment; and forming agate electrode by processing the conductive layer so that a top surfaceof the third insulating layer is exposed and a top surface of theprocessed conductive layer is substantially flush with the top surfaceof the third insulating layer, wherein the second treatment is performedso that a resistance of only a region in the oxide semiconductor layeris increased, wherein the region does not overlap with the thirdinsulating layer, and wherein the first treatment is performed beforeforming any layer over and in contact with the oxide semiconductorlayer.
 2. The method for manufacturing a semiconductor device accordingto claim 1, wherein the region is a channel region of a transistor. 3.The method for manufacturing a semiconductor device according to claim1, wherein heat treatment at a temperature higher than or equal to 450 °C. and lower than or equal to 740 ° C. is performed in the firsttreatment.
 4. The method for manufacturing a semiconductor deviceaccording to claim 1, further comprising the step of forming a fourthinsulating layer comprising the insulating surface, wherein the fourthinsulating layer contains excess oxygen.
 5. The method for manufacturinga semiconductor device according to claim 1, further comprising the stepof forming a fifth insulating layer over and in contact with the oxidesemiconductor layer after performing the first treatment, wherein thefifth insulating layer contains excess oxygen.
 6. A method formanufacturing a semiconductor device, comprising the steps of: formingan oxide semiconductor layer over an insulating surface; performing afirst treatment so that a resistance of the oxide semiconductor layer isreduced; forming a first insulating layer over the oxide semiconductorlayer after performing the first treatment; forming a sacrificial layeroverlapping with a part of the oxide semiconductor layer with the firstinsulating layer interposed therebetween; forming a second insulatinglayer over the sacrificial layer; removing a part of the secondinsulating layer and a part of the sacrificial layer so that a topsurface of the sacrificial layer is exposed and the top surface of thesacrificial layer is substantially flush with a top surface of the restof the second insulating layer; removing the sacrificial layer, therebyforming a third insulating layer which does not cover a part of thefirst insulating layer; performing a second treatment after removing thesacrificial layer; forming a conductive layer over the first insulatinglayer and the third insulating layer after performing the secondtreatment; and forming a gate electrode by processing the conductivelayer so that a top surface of the third insulating layer is exposed anda top surface of the processed conductive layer is substantially flushwith the top surface of the third insulating layer, wherein the secondtreatment is performed so that a resistance of only a region in theoxide semiconductor layer is increased, wherein the region does notoverlap with the third insulating layer, wherein the first treatment isperformed before forming any layer over and in contact with the oxidesemiconductor layer, and wherein plasma oxidation or implantation ofoxygen ions is performed in the second treatment.
 7. The method formanufacturing a semiconductor device according to claim 6, wherein theregion is a channel region of a transistor.
 8. The method formanufacturing a semiconductor device according to claim 6, wherein heattreatment at a temperature higher than or equal to 450 ° C. and lowerthan or equal to 740 ° C. is performed in the first treatment.
 9. Themethod for manufacturing a semiconductor device according to claim 6,further comprising the step of forming a fourth insulating layercomprising the insulating surface, wherein the fourth insulating layercontains excess oxygen.
 10. The method for manufacturing a semiconductordevice according to claim 6, further comprising the step of forming afifth insulating layer over and in contact with the oxide semiconductorlayer after performing the first treatment, wherein the fifth insulatinglayer contains excess oxygen.
 11. A method for manufacturing asemiconductor device, comprising the steps of: forming an oxidesemiconductor layer over an insulating surface; performing a firsttreatment so that a resistance of the oxide semiconductor layer isreduced; forming a first insulating layer over the oxide semiconductorlayer after performing the first treatment; forming a sacrificial layeroverlapping with a part of the oxide semiconductor layer with the firstinsulating layer interposed therebetween; forming a second insulatinglayer over the sacrificial layer; removing a part of the secondinsulating layer and a part of the sacrificial layer so that a topsurface of the sacrificial layer is exposed and the top surface of thesacrificial layer is substantially flush with a top surface of the restof the second insulating layer; removing the sacrificial layer, therebyforming a third insulating layer which does not cover a part of thefirst insulating layer; performing a second treatment after removing thesacrificial layer; forming a conductive layer over the first insulatinglayer and the third insulating layer after performing the secondtreatment; and forming a gate electrode by processing the conductivelayer so that a top surface of the third insulating layer is exposed anda top surface of the processed conductive layer is substantially flushwith the top surface of the third insulating layer, wherein the secondtreatment is performed so that a resistance of only a region in theoxide semiconductor layer is increased, wherein the region does notoverlap with the third insulating layer, wherein the first treatment isperformed before forming any layer over and in contact with the oxidesemiconductor layer, and wherein a laser process is performed in thefirst treatment.
 12. The method for manufacturing a semiconductor deviceaccording to claim 11, wherein the region is a channel region of atransistor.
 13. The method for manufacturing a semiconductor deviceaccording to claim 11, further comprising the step of forming a fourthinsulating layer comprising the insulating surface, wherein the fourthinsulating layer contains excess oxygen.
 14. The method formanufacturing a semiconductor device according to claim 11, furthercomprising the step of forming a fifth insulating layer over and incontact with the oxide semiconductor layer after performing the firsttreatment, wherein the fifth insulating layer contains excess oxygen.